REESE  LIBRARY 

OK  mi. 

UNIVERSITY  OF  CALIFOPx 


n — n — rt, 


NIA 


QUANTITATIVE 
CHEMICAL    ANALYSIS 

BY 

ELECTROLYSIS 


BY 

DR.    ALEXANDER    CLASSEN 

PRIVY  COUNCILLOR    "' 

Professor  of  Petrochemistry  and  Inorganic   Chemistry  in   the   Royal   School   of 
Technology  at  Aachen 

IN  CO-OPERATION  WITH 

DR.    WALTER    LOB 

Lecturer  orrflectrochemistry  in  the  Royal  School  of  Technology  at  Aachen 


AUTHORIZED    TRANSLATION 

TH1D  ENGLISH  FROM  THE  REVISED  AND  GREATLY 
ENLARGED  FOURTH  GERMAN  EDITION 

BY 

WILLIAM  HALE   HERRICK,   A.M. 

Formerly  Profeor  of  Chemistry  in  Iowa  College  and  in  the  Pennsylvania  State  College 

AND 

BERTRAM   B.   BOLTWOOD,   PH.D. 

Instructor  in  j,alytical  Chemistry  in  the  Sheffield  Scientific  School  of  Yale  University 


NEW   YORK 

JOHN   WILEY   &   SONS 

LONDON  :    CHAPMAN  &  HALL,   LIMITED 

1898 


QUANTITATIVE 
CHEMICAL    ANALYSIS 

BY 

ELECTROLYSIS 


BY 

DR.    ALEXANDER    CLASSEN 

PRIVY  COUNCILLOR    '^ 

Professor  of  Electrochemistry  and  Inorganic   Chemistry  in   the   Royal   School   of 
Technology  at  Aachen 

IN  CO-OPERATION  WITH 

DR.    WALTER    LOB 

Lecturer  on  Electrochemistry  in  the  Royal  School  of  Technology  at  Aachen 


AUTHORIZED    TRANSLATION 

THIRD  ENGLISH  FROM  THE  REVISED  AND  GREATLY 
ENLARGED  FOURTH  GERMAN  EDITION 


WILLIAM  HALE   HERRIOK,   A.M. 

Formerly  Professor  of  Chemistry  in  Iowa  College  and  in  the  Pennsylvania  State  College 

AND 

BERTRAM   B.   BOLTWOOD,   PH.D. 

Instructor  in  Analytical  Chemistry  in  the  Sheffield  Scientific  School  of  Yale  University 


NEW   YORK 

JOHN   WILEY   &   SONS 

LONDON  :    CHAPMAN  &  HALL,   LIMITED 

1898 


QTM15 
C  (o 


Copyright,  1898, 

BY 

WILLIAM  HALE  HERRICK 

AND 

BERTRAM  B.  BOLTWOOD. 
f  32. 


BOBKRT   DRUMMOND,   ELECTROTYPER  AND   PRINTER,   NEW  YORK. 


PREFACE  TO  FOURTH   EDITION. 


THE  present  edition,  revised  with  the  assistance  of  Dr. 
Lob,  differs  from  the  previous  editions  in  that  the  Introduc- 
tion has  been  augmented  by  the  insertion  of  a  section  devoted 
to  theory.  This  was  made  the  more  necessary,  since  the  in- 
vestigations of  recent  years  have  been  chiefly  devoted  to  the 
explanation  of  the  reactions  in  solutions,  and  the  determina- 
tion of  the  electrical  magnitudes.  The  necessity  of  specific 
directions  concerning  electrode  tension,  current  strength  and 
decomposition  tension  has  been  demonstrated.  The  author, 
with  the  co-operation  of  his  assistants,  has  experimentally 
determined  these  electrical  magnitudes,  not  only  for  his  own 
methods  for  the  determination  and  separation  of  the  metals, 
but  also  for  a  number  of  other  methods,  and  has  incorporated 
them  in  the  text.  Additional  methods  by  other  authors,  in 
which  directions  concerning  these  important  factors  are  want- 
ing, have  been  omitted,  in  consideration  of  the  fact  that  these 
are  either  uncertain  or  entirely  impractical ;  mention  of  them 
has,  however,  been  made  in  the  references  to  the  literature. 

The  book  has  been  made  more  complete  by  the  description 
of  various  measuring  instruments,  sources  of  current  and 
apparatus ;  together  with  an  explanation  of  simple  and  com- 
plete appliances  for  carrying  out  electrolytic  experiments. 
These  have  been  illustrated  by  a  large  number  of  new  cuts, 
in  the  text  and  in  the  appended  tables. 

The  publishers  have  spared  neither  pains  nor  expense  to 
make  these  new  illustrations  as  perfect  as  possible;  I  feel 
called  upon  to  express  here  my  full  appreciation  of  this  fact. 

A.  CLASSEN. 
AACHEN,  January  18,  1897. 

iii 


TRANSLATORS1  PREFACE. 


THE  Author's  Preface  to  the  Fourth  Edition  points  out  so 
fully  the  improvements  over  preceding  editions,  that  the 
translators  need  add  nothing.  It  is  plainly  a  more  complete, 
scientific  and  logically  arranged  work  than  heretofore. 

The  translators  have  made  some  additions,  as  had  their 
senior  in  previous  editions;  and  have  corrected  some  errors 
in  the  German  edition,  apparently  the  result  of  hasty  com- 
pilation. The  "  Special  Part"  of  former  German  editions 
(which  is  omitted  in  the  fourth)  has  been  retained  in  the 
form  of  an  appendix,  and  has  been  revised  and  brought  up  to 
date.  In  addition  to  this  a  carefully  prepared  index  has  been 
added,  and  the  translators  believe  that  the  value  and  con- 
venience of  the  work  is  thereby  much  enhanced. 

WILLIAM  HALE  HERRICK. 
BERTRAM  B.   BOLTWOOD. 
January,  1898. 


CONTENTS. 


SECTION  I. 
GENERAL    PART. 

PAGE 

INTRODUCTION 1 

ION  THEORY  6 

FARADAY'S  LAW 10 

OHM'S  LAW  12 

TENSION  AND  ITS  SIGNIFICANCE 13 

SIGNIFICANCE  OF  CURRENT  STRENGTH 17 

SIGNIFICANCE  OF  RESISTANCE  19 

THEORY  OF  ELECTROLYTIC  PRECIPITATION , 21 

DETERMINATION  OF 'THE  CURRENT  MAGNITUDES  : 

1.  MEASUREMENT  OF  THE  CURRENT  STRENGTH 23 

Oxyhydrogeu-gas  Voltameter 24 

Weight  Voltameter  26 

Tangent  Galvanometer 26 

Sine  Galvanometer 28 

Other  Forms  of  Galvanometers 29 

Spring  Galvanometer 31 

Amperemeter 31 

2.  MEASUREMENT  OF  THE  TENSION 32 

Voltmeter ....  32 

Torsion  Galvanometer  33 

Lippmanu  Capillary  Electrometer 35 

Quadrant  Electrometer 37 

SOURCES  OF  CURRENT 38 

PRIMARY  GALVANIC  ELEMENTS  : 

Leclanche  Cell 39 

Meidinger  Cell 41 

Daniell  Cell 43 

Gravity  Cell 43 

vii 


viii  CONTENTS. 

PAGE 

Grove  Cell  44 

Bunsen  Cell 45 

Cupron  Element 46 

Edison-Lalande  Element 47 

SECONDARY    GALVANIC    ELEMENTS   (ACCUMULATORS,    OR    STORAGE 

BATTERIES) ...     47 

General  Rules  for  the  Handling  of  Accumulators 54 

PHYSICAL  METHODS  OF  PRODUCING  THE  CURRENT  : 

Electromagnetic  Machines 60 

Thermo-electric  Piles 64 

REGULATION  OF  THE  CURRENT 73 

PROCESS  OF  ANALYSIS 83 

HISTORICAL ..  101 

ARRANGEMENTS  FOR  ANALYSIS 107 

Arrangement  for  Smaller  Experiments 108 

Former  Equipment  of  the  Electrochemical  Institute  at  Aachen.. . .  Ill 
Present  Equipment  of  the  Electrochemical  Institute  at  Aachen. ...  124 


SECTION  II. 
SPECIAL    PART. 

QUANTITATIVE    DETERMINATION    OF   THE   METALS. 

IRON 137 

COBALT 141 

NICKEL 143 

ZINC 144 

MANGANESE 148 

ALUMINIUM,  URANIUM,  CHROMIUM,  BERYLLIUM 153 

COPPER 153 

BISMUTH 162 

CADMIUM , 163 

LEAD . 166 

THALLIUM 170 

SILVER 172 

MERCURY 174 

GOLD 177 

ANTIMONY 178 

PLATINUM 182 

PALLADIUM 183 

TIN..  .  183 


CONTENTS. 


PAGE 

ARSENIC.  .  .  ......  ......  ...........  .   .  .................  ...........  188 

POTASSIUM,  AMMONIUM  (NITROGEN)  ........  ...  .....................  188 

DETERMINATION  OF  NITRIC  ACID  IN  NITRATES  ............  .  .......  189 


DETERMINATION    OF   THE    HALOGENS. 

CHLORINE,  BROMINE,  IODINE  ......................  .  ..............  190 

SEPARATION   OF   THE    METALS. 

IRON  ..................................................  .  ......   191-204 

Iron—  Cobalt  ..................................................  191 

Iron  —  Nickel  ......  ...........................................  192 

Iron  —  Zinc  ..................................................  193 

Iron  —  Manganese  .......................  ......................   194 

Iron  —  Aluminium  .............................................   196 

Iron  —  Uranium  ...............................................   198 

Iron  —  Chromium  ..........................................  ...  199 

Iron—  Aluminium—  Chromium  .......  .  .........................  200 

Iron—  Chromium—  Uranium  ............................  .  .......  200 

Iron  —  Beryllium  .  .............................................  201 

Iron—  Beryllium  —  Aluminium  ..................................  201 

Iron  —  Copper  .................................................  202 

Iron—  Lead  ...................................................  204 

COBALT  ......................................................  204-206 

Cobalt—  Zinc  ...............................................  204 

Cobalt—  Aluminium  ...........................................  204 

Cobalt  —  Uranium  ............................................  204 

Cobalt  —  Chromium  .  .  ........................................  204 

Cobalt  —  Uranium  —  Chromium  .............  ....................  204 

Cobalt  —  Copper  ...............................................  205 

Cobalt—  Bismuth  ........  ...  ..................................  205 

Cobalt—  Lead  ................................................  206 

Cobalt  —  Mercury  .......  .  ......................................  206 

NICKEL  .......  .  ............  .  ..............................  206,207 

Nickel—  Manganese  .........................................  206 

Nickel  —  Aluminium  .................  '.  .........................  206 

Nickel  —  Uranium   ......  ,  .....................................  206 

Nickel  —  Chromium  .........  ..................................  206 

Nickel—  Copper  ..............................................  206 

Nickel—  Lead  ...............................................  207 

Nickel—  Mercury  ...................  .........................  207 


S  CONTENTS. 

PAGE 

ZINC 208-211 

Zinc — Manganese 208 

Zinc — Aluminium 208 

Zinc— Copper 208 

Zinc— Cadmium 209 

Zinc— Lead 210 

Zinc— Silver 210 

Zinc— Mercury 210 

MANGANESE 211,  212 

Manganese — Copper 211 

Manganese — Cadmium 212 

COPPER 3i2-21? 

Copper — Cadmium 212 

Copper — Lead 213 

Copper— Silver 215 

Copper— Mercury 216 

Copper — Arsenic 217 

CADMIUM  217,218 

Cadmium— Lead 217 

Cadmium — Mercury 218 

LEAD 218,219 

Lead— Silver 218 

Lead— Mercury 218 

Lead— Antimony 219 

SILVER 220 

Silver— Antimony , 220 

Silver— Arsenic 220 

MERCURY 220,  221 

Mercury — Antimony 220 

Mercury — Arsenic 221 

ANTIMONY 221-228 

Antimony— Tin 221 

Antimony — Arsenic 224 

Antimony — Tin — Arsenic 225 

TIN — PHOSPHORIC  ACID 228 

PLATINUM — IRIDIUM 228 

SEPARATION  OF  GOLD  FROM  OTHER  METALS 228 

POTASSIUM— SODIUM 229 

SODIUM— AMMONIUM.  .  .  229 


CONTENTS.  XI 


APPENDIX. 

SOME  APPLIED  EXAMPLES  OF  ELECTROCHEMICAL 
ANALYSIS. 

PAGE 

BRASS 231 

SILVER  COIN  233 

NICKEL  COIN 233 

GERMAN  SILVER 234 

BRONZE , 235 

PHOSPHOR-BRONZE 235 

MANGANESE  PHOSPHOR-BRONZE 236 

SOLDER 236 

WOOD'S  METAL 237 

HARD  LEAD,  TYPE-METAL. 237 

ALLOY  OF  ANTIMONY  AND  TIN 238 

ALLOY  OF  ANTIMONY  AND  ARSENIC 238 

ALLOY  OF  ANTIMONY,  TIN  AND  ARSENIC 239 

SPATHIC  IRON-ORE 239 

HEMATITE  240 

LIMONITE 241 

CLAY  IRON-ORE , 242 

BOG  IRON-ORE  242 

CHROME  IRON-ORE  242 

PSILOMELANE 244 

SPHALERITE  (ZINC  BLENDE) 247 

CALAMINE  AND  SMITHSONITE 249 

ULTRAMARINE , 249 

REFINERY  SLAG 250 

COPPER  AND  LEAD  SLAGS 250 

BLAST-FURNACE,  CUPOLA,  AND  BESSEMER  SLAGS 252 

ZIRCON 253 

ARSENOPYRITE 253 

CHALCOPYRITE  254 

NICKEL  MATTE,  COPPER  MATTE 255 

COPPER  SPEISS,  LEAD  SPEISS  . .   256 

PYRARGYRITE 257 

TETRAHEDRITE 257 

FURNACE  "Sows" - 258 

STIBNITE  (ANTIMONY  GLANCE). 259 

ULLMANITE 259 

BOURNONITE .  260 


Xll  CONTENTS. 

PAGE 

ZlNKENITE 260 

LlNN^EITE 261 

CoBALTITE    .  » 261 

COBALTIFEROUS  ARSENOPYRITE 262 

CERUSSITE 263 

GALENA ... 263 

PYROMORPHITE 264 

LEAD  MATTE 264 

CINNABAR , 265 

SOFT  LEAD  (CRUDE  LEAD) 265 

ANTIMONY 268 

SPELTER  (CRUDE  ZINC) 268 

BLISTER  COPPER 269 

REFINED  COPPER 272 

TIN  . 272 

SILVER . .  273 

COMMERCIAL  NICKEL 274 

PIG  IRON,  STEEL,  SPIEGEL,  FERROMANGANESE 275 

TABLES  FOR  CALCULATION  OF  ANALYSES 280-283 

REAGENTS 284-287 

POTASSIUM  OXALATE 284 

AMMONIUM  OXALATE 284 

OXALIC  ACID 285 

AMMONIUM  SULPHATE 285 

SODIUM  SULPHIDE 285 

ALCOHOL 286 

INDEX  OF  AUTHORS 287 

INDEX  OF  SUBJECTS..  293 


QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS. 


PAET    L-GEKEKAL    PAET. 


INTRODUCTION.* 

WATER  acidified  with  sulphuric  acid  is  decomposed  into 
its  elements,  hydrogen  and  oxygen,  when  a  galvanic  current 
is  passed  through  it;  a  large  number  of  compound  sub- 
stances conduct  themselves  in  a  similar  manner.  This  gal- 
vanic decomposition  is  called  electrolysis,  and  the  substances 
which  are  decomposed  by  the  electric  current  are  known  as 
electrolytes.  The  substances  into  which  electrolytes  are 
separated  by  the  electric  current  are  naturally  divided  into 
two  groups :  Those  which  separate  at  the  positive  electrode, 
or  anode  (connected  with  the  +  pole  of  the  source  of  the 
current),  and  which  are  therefore  the  electro-negative  con- 
stituents, are  called  anions;  those  which  separate  at  the 
negative  electrode,  or  cathode  (connected  with  the  —  pole 
of  the  source  of  the  current),  the  electro-positive  constitu- 
ents, are  called  cathions. 

The  metalloids,  or  electro-negative  acid  groups,  therefore 
appear  at  the  positive  electrode,  while  the  metals  are  sepa- 
rated at  the  negative  electrode. 

*  An  elementary  knowledge  of  galvanic  action  is  assumed. 


2  QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 

For  instance,  if  the  electric  current  is  passed  through 
the  solution  of  a  haloid  salt,  the  halogen  is  separated  at  the 
anode,  the  metal  at  the  cathode. 

CuCl2  =  C12  +  Cu, 
ZnCl2  =  C12  +  Zn. 

Oxygen  salts  act  in  a  similar  manner. 

CuS04        =  S04        +  Cu, 
Cu(N03)2  =  (N03)2  +  Cu. 

Many  acids  are  decomposed  in  a  similar  manner.* 

H2S04  =  S04  +  H2, 
2HC1    =  C12  +  H2. 

The  substances  formed  by  electrolytic  decomposition, 
however,  generally  undergo  further  chemical  change,  or  are 
acted  on  by  the  electrodes ;  various  secondary  reactions  take 
place. 

In  the  electrolysis  of  a  solution  of  copper  sulphate  between 
platinum  electrodes,  the  secondary  process  consists  in  the  re- 
action with  water  of  the  group  SO4,  which  cannot  exist 
uncombined. 

SO4  +  H2O  =  H2S04  +  O. 

The  evolution  of  oxygen  gas,  which  is  partially  due  to  this 
secondary  reaction,  is  observed  at  the  positive  pole.  Pri- 
marily the  water  itself  splits  off  oxygen  in  the  electrolysis  of 
aqueous  solutions. 

*  Some  acids  are  not  decomposed  by  the  electric  current ;  e.g.,  silicic, 
carbonic,  and  boric  acids. 


INTRODUCTION.  3 

In  the  electrolysis  of  hydrochloric  acid,  the  chlorine  set 
free  at  the  anode  reacts  with  water,  forming  hypochlorous 
acid,  chloric  acid,  perchloric  acid,  etc.  Similar  secondary 
reactions  are  observed  in  the  electrolysis  of  chlorides.  If  a 
solution  of  ammonium  chloride,  for  example,  is  submitted  to 
electrolysis  the  nascent  chlorine  acts  on  the  un  decomposed 
salt,  with  the  production,  among  other  substances,  of  nitro- 
gen, or  nitrogen  chloride.  Haloid  salts  of  the  alkaline  earths 
show  similar  phenomena. 

Nitric  acid,  on  electrolysis,  gives  in  the  first  place 

SHINTO,  =  4H2  (cathion)  +  8E"O3  (anion). 
The  latter  then  splits  up  further: 

4N2O6  =  4^1,0.  +  2O2  (anion). 

The  oxygen  is  given  off,  while  the  anhydride  forms  nitric 
acid  again  with  water: 

4N  A  +  4H2o  =  SHNO,. 

The  hydrogen,  on  the  contrary,  which  appears  as  cathion, 
is  not  set  free  but  acts  reducingly  on  the  nitric  acid  present  : 


4H,  +  mros  =  NH3  +  3HaO. 

In  the  presence  of  sulphuric  acid,  or  a  sulphate,  this  de- 
composition is  complete,  the  final  product  being  ammonium 
sulphate. 

This  decomposition  of  nitric  acid  is  of  practical  importance 
in  chemical  analysis.  From  a  nitric  acid  solution  which  con- 
tains copper  and  zinc,  the  former  metal  only  is  reduced  ;  this 
fact  can  be  utilized  for  the  separation  of  the  two  metals.  If, 


4  QUANTITATIVE   ANALYSIS   BY    ELECTROLYSIS. 

now,  the  current  is  allowed  to  pass  for  a  long  time  after  the 
reduction  of  the  copper,  the  nitric  acid  is  gradually  converted 
into  ammonia,  and  the  zinc  then  separates  from  the  solution. 
If  the  salts  of  metals  which  decompose  water  at  ordinary 
temperatures  (alkalies  and  alkaline  earths)  are  electrolyzed, 
secondary  reactions  occur  at  the  negative  electrode : 

K2SO4  =  SO4  (anode), 

K2  (cathode), 

(S04  +  H20    =  H2S04  +  0), 
Ka     +  2H.O  =  2KOH  +  Ha. 

The  decomposition  products,  then,  are  sulphuric  acid  and 
oxygen  at  one  electrode,  potassium  hydroxide  and  hydrogen 
at  the  other.  It  must  be  borne  in  mind,  however,  that  a 
small  portion  of  the  hydrogen  and  oxygen  formed  owes  its 
existence  to  the  primary  electrolysis  of  the  water. 

The  metals  disengaged  at  the  negative  electrode  may  yield 
secondary  products  by  acting  on  the  substances  in  solution. 
So,  for  instance,  in  the  electrolysis  of  cupric  chloride,  the 
separated  copper  reacts  with  the  cupric  chloride  to  form 
cuprous  chloride;  copper  acetate  yields,  at  the  cathode,  a 
mixture  of  copper  and  cupric  (or  cuprous)  oxide. 

In  the  electrolysis  of  organic  compounds,  the  groups  set 
free  at  an  electrode  may  be  decomposed  in  a  manner  analo- 
gous to  that  noted  in  inorganic  compounds,  and  yield  various 
products. 

The  electrolysis  of  potassium  acetate  should  yield,  as  iinal 
products,  potassium  (potassium  hydroxide)  and  acetic  acid. 

Instead  of  this,  the  acetic  acid  splits  either  into  carbon 
dioxide  and  ethane,  or  ethylene  is  formed  by  the  action  of 
oxygen  on  the  ethane. 

Potassium   valerate   yields,   in    addition    to  valeric  acid, 


INTRODUCTION.  5 

carbon  dioxide  and  octane;  the  latter  is  oxidized  by  con- 
tinued electrolysis  to  isobutylene  and  water. 

Sodium  snccinate  yields,  among  other  products,  ethylene 
and  carbon  dioxide ;  potassium  lactate  breaks  up  into  carbon 
dioxide  and  acetaldehyde. 

For  the  purposes  of  quantitative  chemical  analysis,  only 
such  solutions  are  adapted,  as  indicated  by  the  foregoing,  as 
are  decomposed  completely  by  the  current  without  the  forma- 
tion of  injurious  intermediate  products.  Solutions  which 
contain  a  free  inorganic  acid  are  well  adapted  to  electrolysis, 
because  of  their  high  conductivity. 

Of  all  compounds  of  the  metals,  the  double  oxalates  are 
the  best  adapted  to  quantitative  analysis.*  Oxalic  acid  is 
decomposed  by  the  electric  current : 

C3H2O4  =  2CO2  (anode), 
H2  (cathode). 

When  potassium  oxalate  is  subjected  to  electrolysis,  the 
principal  decomposition -products  are: 

K3C,O4  =  2CO3  (anode), 

K2  (cathode), 

K3          +  2HaO  =  2KOH  +  H2  (cathode), 
2KHO  +  2CO,  =  2KHCO3. 

When  ammonium  oxalate  is  used,  the  decomposed  solu- 
tion contains  hydrogen  ammonium  carbonate.  The  latter 
partly  decomposes  into  ammonia  and  carbon  dioxide. 

In  the  electrolysis  of  double  oxalates,  e.g.,  of  zinc  ammo- 


*  Classen,  Ber.  d.  ch.  Ges.,  14,  1622,  2771;  17,  2467;  18,  1104,  1687;  19, 
323  ;  20,  504  ;  21,  2900. 


6  QUANTITATIVE  ANALYSIS    BY   ELECTROLYSIS. 

nium  oxalate,  decomposition  takes  place  as  follows  :  Zinc 
oxalate  breaks  up  into  zinc  and  carbon  dioxide,  and  ammo- 
nium oxalate  into  ammonium  and  carbon  dioxide.  The  car- 
bon dioxide,  which  separates  at  the  positive  pole,  combines 
with  the  ammonium  to  form  hydrogen  ammonium  carbonate, 
as  above  explained. 

In  the  decomposition  of  oxalates  there  are  no  unfavora- 
ble secondary  reactions.  All  oxalates  are  decomposed  by  the 
electric  current  with  greater  or  less  ease,  and  the  reduced 
metals  are  not  attacked  by  the  decomposition-products,  even 
when  the  current  becomes  weaker  during  the  reaction.  When 
the  reaction  is  complete,  the  solution  can  be  poured  off  at 
once,  and  the  weight  of  the  separated  metal  determined.  (See 
further  details  later.) 

THE  ION  THEORY. 

Before  proceeding  to  a  description  of  the  appliances  and 
methods  of  quantitative  analysis,  the  reactions  which  take 
place  in  solutions  during  their  decomposition,  together  with 
the  magnitudes  which  here  come  into  consideration,  should 
be  made  perfectly  clear. 

The  ion  theory,  proposed  in  connection  with  the  researches 
of  van't  Hoff,  by  the  Swedish  investigator  Arrhenius,  furnishes 
us  with  a  comprehensive  picture  of  the  same.  According  to 
this  theory,  a  partial  splitting  up  of  the  dissolved  compounds 
into  their  component  parts  takes  place  in  aqueous  solutions ; 
a  dissociation  which,  in  contradistinction  to  the  ordinary,  is 
called  electrolytic  dissociation. 

In  tlie  case  of  a  sodium  chloride  solution,  for  example, 
many  phenomena,  such  as  the  osmotic  pressure,  the  lowering 
of  the  freezing- point,  and  others,  necessitate  the  assumption 
that,  besides  the  particles  of  undecomposed  Nad,  separate. 


THE   ION   THEORY.  7 

particles  of  "N&  and  Cl  are  present  in  the  solution.  The 
latter  are  entirely  different  from  atomic  Na  and  Cl,  since  it 
is  of  course  impossible  to  conceive  of  a  Na  atom,  which  reacts 
violently  with  water,  as  existing  free  in  an  aqueous  solution. 

The  difference  between  these  electrolytic  dissociation 
products  and  atoms  lies  in  an  unlike  content  of  energy.  This 
of  course  materially  affects  the  other  properties. 

While  the  atom  in  itself  must  be  considered  non-electric 
(containing  as  much  positive  as  negative  electricity),  it  is 
necessary  to  attribute  a  certain  electric  charge  to  the  products 
of  electrolytic  dissociation.  These  electrically  charged  par- 
ticles are  called  ions  (iovres,  the  wandering),  a  name  given 
to  them  by  Faraday. 

The  phenomena  of  the  osmotic  pressure  and  the  depression 
of  the  freezing-point,  already  mentioned,  have  identified  with 
electrolytic  dissociation,  and  accordingly  with  ions,  certain 
classes  of  chemical  compounds,  namely,  acids,  bases,  and 
salts,  but  not  indifferent  organic  compounds.  Since  it  has 
been  shown  that  the  former  compounds,  and  indeed  only  these 
and  no  others,  conduct  the  electric  current  in  aqueous  solu- 
tions, the  existence  of  ions  and  the  characteristic  of  being  decom- 
posed by  the  electric  current  have  been  brought  into  causal 
relation. 

The  substances  which  are  electrolytically  dissociated  in 
solution,  and  therefore  conduct  the  electric  current,  are  called 
electrolytes ;  those  which  are  not  dissociated  into  ions  and  do 
not  permit  the  passage  of  the  current,  non-electrolytes. 
Acids,  bases,  and  salts  are  accordingly  electrolytes ;  all  other 
substances,  such  as  chloroform,  benzene,  ether,  sugar,  etc., 
non-electrolytes. 

Relative  to  the  formation  of  ions,  all  acids  exhibit  a  com- 
mon characteristic  in  yielding  hydrogen  ions ;  correspondingly 
all  bases  give  hydroxyl  ions. 


QUANTITATIVE   ANALYSIS   BY    ELECTROLYSIS. 

An  acid  is  accordingly  dissociated  as  follows : 


HC1 

into 

H 

Cl    . 

H2S04 

into 

H2 

S04 

HN03 

into 

H 

N03 

C204H2 

into 

H2 

C,04 

CH3COOH 

into 

H 

CH3COO,  etc. 

By  the  electrolytic  dissociation  of  acids,  therefore,  all  of 
those  hydrogen  atoms,  which  are  replaceable  by  metals  on 
neutralization  with  bases,  are  brought  into  the  state  of  ions ; 
at  the  same  time  the  corresponding  acid  radicals  pass  over 
into  the  ionic  condition. 

The  dissociation  of  bases  is  analogous : 

JSaOH  into         Na       OH 

NH4OH        into         NH4     OH 
Ca(OH)2         into         Ca       (OH),,  etc. 

All  the  hydroxyl  groups,  which  are  replaceable  by  acid 
radicals  on  neutralization  with  acids,  change  to  the  ion  con- 
dition; and  simultaneously  the  basic  radicals,  i.e.,  the  metals. 

Salts  are  accordingly  dissociated  into  metal  and  acid  ions. 

When  the  current  passes,  a  part  of  the  ions  migrate  to  the 
positive  electrode,  a  part  to  the  negative  electrode.  Since  the 
ions  possess  an  electric  charge,  those  attracted  by  the  positively 
charged  electrode  must  be  negatively  charged  (anions),  and 
those  attracted  by  the  negative  electrode  positively  charged 
(cat/lions). 

Hydrogen  and  all  metals  are  electro-positive ;  halogens  and 
acid  radicals,  electro-negative.  The  former  are  cathions,  the 
latter  anions.  Therefore  when  a  salt  of  a  metal  is  electrolyzed. 
the  metal  separates  at  the  negative  electrode,  the  acid  radical 
at  the  positive  electrode. 


THE   ION   THEORY/S^f  TAH     a«i\K^^      9 


It  h&s  been  demonstrated  that  the  electrolytic  dissociation 
increases  with  the  dilution,  and  proportionally  to  it  the  elec- 
trical conductivity  of  the  ions  also.  Arrhenius  therefore  drew 
the  conclusion  that  the  ions  alone  conduct  the  current,  and  that 
the  undissociated  portion  takes  no  part  in  the  electrolysis. 
This  assumption  has  been  proved  correct,  and  through  it  elec- 
trolytic dissociation  presents  an  entirely  different  aspect. 

The  primary  products  which  are  set  free  at  the  electrodes 
are  not  separated  by  the  current ,  but  existed  previously  in 
the  solution  in  the  form  of  cathions  and  anions. 

Since  the  substances  separate  at  the  electrodes  in  an  atomic, 
non-electric  condition,  while  the  ions  possess  an  electric  charge, 
a  discharge  of  the  ions  must  take  place  at  the  electrodes.  Such 
is  in  fact  the  case.  The  negatively  charged  electrode  attracts 
the  positively  charged  cathions,  and  on  their  coming  into 
contact  a  neutralization  of  equivalent  parts  of  positive  and 
negative  electricities  takes  place,  accompanied  by  the  dis- 
appearance of  electricity.  The  separation  of  the  substance  in 
an  atomic  form  is  then  possible.  The  work  done  by  the  elec- 
tric current  consists  in  the  attraction  and  discharge  of  the  ions, 
but  not  in  the  decomposition  of  the  dissolved  compound. 

What  has  been  said  of  the  cathion  naturally  applies  in  a 
similar  manner  to  the  anion. 

The  degree  of  dissociation  at  a  certain  concentration  [and 
temperature]  is  a  fixed  magnitude  for  every  substance.  Here 
the  objection  might  be  raised,  for  example,  that  in  a  copper 
sulphate  solution  in  which  ^  of  the  copper  sulphate  is  split  up 
into  Cu  and  SO4  ions  and  f  is  present  as  undissociated  salt, 
the  electrolysis  must  come  to  a  standstill  after  the  third  of  the 
copper  has  been  removed,  since  there  are  no  more  copper 
ions  present.  Experiment  shows  that  all  the  copper  may  be 
separated.  This  phenomenon  is  explained  by  the  law  of  mass 
action,  according  to  which  the  product  of  the  ion  con- 


10  QUANTITATIVE   ANALYSIS   BY   ELECTROLYSIS. 

centrations  remains  constant  at  a  fixed  concentration  of  the 
solution.  In  other  words,  if  copper  ions  disappear  from  the 
solution  owing  to  the  removal  of  the  metal,  the  undissociated 
salt  present  dissociates  and  furnishes  new  copper  ions.  This 
process  continues  until  all  the  copper  ions  have  been  removed 
by  precipitation  as  atoms.  What  is  true  of  copper  sulphate 
is  true  of  all  acids,  bases,  and  salts,  and  anions  behave  similarly 
to  cathions. 

FARADAY'S   LAW. 

This  law,  which  is  named  from  its  discoverer  and  is  the 
basis  of  all  electrolytic  phenomena,  includes  the  two  following 
propositions : 

1.  The  quantities  of  ions  separated  at  the  electrodes  during 
equal    intervals    of    time    are    directly   proportional   to   the 
current  strength. 

2.  Equal  quantities  of  electricity  separate  the  ions  in  pro- 
portion to  their  chemical  equivalent  weights. 

The  truth  of  the  first  statement  may  be  readily  demon- 
strated by  electrolyzing  a  copper  sulphate  solution  for  ten 
minutes  with  a  current  of  certain  strength  and  determining 
the  weight  of  the  separated  copper.  If  now  in  a  second 
operation  a  current  of  twice  the  former  strength  be  passed 
through  the  same  solution  for  an  equal  length  of  time,  the 
weight  of  the  copper  separated  in  the  second  case  will  be 
twice  as  great  as  that  precipitated  in  the  first  experiment. 

The  second  proposition,  called  in  brief  Faraday's  Law, 
is  proved  experimentally  by  passing  the  same  current  simul- 
taneously through  a  series  of  solutions  of  metallic  salts  and 
weighing  the  quantities  of  metals  which  separate.  It  will  then 
be  found  that  the  weights  are  proportional  to  the  chemical 
equivalent  weights  of  the  metals.  Accordingly  solutions  of 
silver  nitrate,  cupric  chloride,  and  ferric  chloride  when  decom- 


FARADAY'S  LAW.  11 

posed  by  the  same  current,  yield  precipitates  of  metals,  the 
weights  of  which  bear  the  following  ratio  to  one  another  : 

108  -63-55'9 

8  -"  -~- 


The  ratio  when  silver  nitrate,  cuprous  chloride,  and  fer- 
rous chloride  are  used  is  correspondingly 

108:63:^, 

so  that  the  equivalent  weight  is  dependent  upon  the  valence 
of  the  metal  in  the  compound  employed.  That  which  may 
be  conveniently  carried  out  in  the  case  of  the  metals,  i.e., 
the  cathions,  holds  true  similarly  in  the  case  of  the  anions. 

The  law  of  Faraday,  regarded  in  the  light  of  the  ion 
theory,  leads  to  a  series  of  new  conclusions.  It  declares  that 
equal  currents  of  electricity  always  separate  equal  quantities 
of  univalent,  half  the  quantities  of  bivalent,  and  one-  third  the 
quantities  of  trivalent  ions.  This  separation  consists  in  the 
discharge  of  the  electrically  charged  ions  ;  therefore  it  follows 
that  equal  quantities  of  univalent  ions  are  the  carriers  of  equal 
electric  charges,  that  the  same  quantities  of  bivalent  ions  bear 
twice,  of  trivalent  ions  three  times,  as  great  charges. 

Consequently  all  univalent  ions,  independent  of  their 
chemical  nature,  bear  equal  quantities  of  electricity,  all  biva- 
lent ions  twice  the  quantity,  etc.  The  magnitude  of  this 
charge  is  considerable.  Experiment  has  shown  that  by  the 
action  of  96,500  coulombs  of  electricity  on  an  electrolyte,  a 
quantity  of  ions  equal  in  grams  to  their  atomic  weight  divided 
by  their  valence  always  passes  over  into  the  atomic  condition, 
or,  as  it  may  be  stated,  by  96,500  coulombs  a  gram  equiva- 
lent of  ions  will  be  discharged,  or,  better,  neutralized  on  the 
electrode.  For  this  neutralization  the  ions  must  carry  a 


12  QUANTITATIVE   ANALYSIS    BY   ELECTROLYSIS. 

quantity  of  electricity  equal,  but  opposite  in  sign,  to  that 
supplied  by  the  source  of  current  to  the  electrodes.  A  gram 
equivalent  of  ions  must  therefore  be  the  bearer  of  96,500 
coulombs.*  This  conclusion,  arrived  at  by  v.  Hehnholtz, 
leads  to  the  assumption  that  every  bond  of  valence  of  an  ele- 
mentary or  complex  ion  is  charged  with  the  same  quantity  of 
positive  or  negative  electricity,  which,  similarly  to  an  electric 
atom,  cannot  be  further  divided. 

OHM'S  LAW. 

Ohm's  Law  holds  good  for  solutions  as  it  does  for  metals: 
C  =  |,     E  =  0-K. 

"When  suitable  units  are  chosen,  the  current  strength  is 
equal  to  the  quotient  of  the  electromotive  force  divided  by 

the  resistance. 

The  ampere  serves  as  the  unit  of  current  strength,  and  is 
that  current  strength  by  which,  in  one  second,  0.328  nag.  of 
copper  will  be  precipitated,  f  The  unit  of  resistance  is  the 
resistance  (at  0°)  of  a  column  of  mercury  having  a  length  of 
106.3  cm.  and  a  cross-section  of  1  sq.  mm.  It  is  called  the 
ohm.  The  unit  of  electromotive  force  is  the  volt,  and  is 
defined  by  the  equation 

1  ampere  X  1  ohm  =  1  volt, 

volt 

ampere  =  —, — . 
ohm 

*  "  The  quantity  of  electricity  necessary  for  the  separation  of  a  gram 
equivalent,  i.e.,  96,540  coulombs,  is  to  be  denoted  by  the  symbol  F,  in 
remembrance  of  Faraday."  Report  of  Commission  on  Electrical  Units, 
Deutsche  Elektrochemische  Gesellschaft.  Zeit.  f.  Elektrochemie,  1897-98, 
p.  36.— Trans. 

f  An  ampere  may  be  also  defined  as  the  strength  of  the  current  which 
flows  through  a  resistance  of  1  ohm  when  the  electromotive  force  is  equal 
to  1  volt.— Trans. 


TENSION    AND    ITS    SIGNIFICANCE.  13 

An  electromotive  force  of  one  volt  with  a  resistance  of  one 
ohm  gives  a  current  strength  of  one  ampere. 

Every  source  of  current  furnishes  a  certain  electromotive 
force ;  it  possesses  a  certain  tension.  If  the  two  poles  of  a 
source  of  current  are  connected  by  a  conductor,  there  takes 
place  along  this  connection  a  fall  of  potential  which  is  pro- 
portional to  its  resistance.  If  an  electrolyte,  into  which  ex- 
tend two  electrodes,  is  included  in  the  circuit,  there  arises  a 
difference  of  potential  between  the  electrodes  which  is  called 
the  electrode  tension.  This  tension  at  the  electrodes  is  of  con- 
siderable importance  in  electrolysis,  since  it  denotes  that  elec- 
tromotive force  which  comes  into  action  in  the  cell  itself. 
Each  of  the  three  factors  given  in  Ohm's  Law  is  of  signifi- 
cance for  quantitative  electrolysis ;  we  will  next  proceed  to 
their  consideration. 


TENSION  AND  ITS  SIGNIFICANCE  FOR  ELECTROLYSIS. 

As  quantitative  electrolysis  is  employed  chiefly  for  the 
determination  of  metals,  it  will  be  wrell  to  here  consider  some 
of  the  general  properties  of  solutions  of  metallic  salts. 

In  accordance  with  present  theory,  the  origin  of  an  electro- 
motive force  is  explained  as  follows :  If  a  strip  of  metal,  zinc 
for  example,  be  dipped  into  an  electrolyte,  say  zinc  chloride, 
the  zinc  ions  present  in  the  solution  will  have  a  tendency  to 
discharge  their  electricity  upon  the  zinc  and  to  pass  over  into 
an  atomic  condition.  This  tendency  may  be  considered  as  a 
pressure  directed  from  the  liquid  toward  the  metal,  and  is 
known  as  the  osmotic  pressure  of  the  ions.  The  metallic  zinc, 
however,  exerts  a  pressure  in  the  opposite  direction,  which  is 
due  to  the  tendency  of  the  zinc  atoms  to  pass  into  the  solution 
and  assume  the  condition  of  ions,  and  is  opposed  to  the  sepa- 
ration of  the  ions  already  present,  which  strive  to  leave  the 


14  QUANTITATIVE   ANALYSIS   BY   ELECTROLYSIS. 

.electrolyte  as  atoms.  This  latter  pressure  is  known  as  the 
electrolytic  solution  pressure. 

Since  the  ions  are  bearers  of  electric  charges,  it  is  evident 
that  the  simultaneous  action  of  these  two  pressure-forces  is 
intimately  connected  with  the  production  of  electricity,  and 
experience  has  taught  that  the  electromotive  force  between 
the  liquid  and  the  metal  may  be  considered  a  function  of  these 
two  pressures. 

Osmotic  pressure  of  the  ions  and  electrolytic  solution  pres- 
sure cause  currents  in  opposite  directions.  The  positively 
charged  ions  of  the  salt  solution  tend,  as  a  result  of  the  osmotic 
pressure,  to  give  up  their  charges  to  the  exposed  metal  and  to 
charge  this  positive ;  while,  on  the  other  hand,  the  electrolytic 
solution  pressure  forces  positive  ions  out  from  the  metal  and 
into  the  solution,  leaving  an  equivalent  negative  charge  upon 
the  metal  itself. 

If  a  similar  course  of  reasoning  be  applied  to  the  case 
where  an  electric  current  is  passed  between  two  platinum 
electrodes  which  dip  into  a  solution  of  a  metallic  salt,  the 
following  conditions  will  be  recognized.  A  definite  difference 
of  potential  will  exist  between  the  electrodes,  as  a  result  of 
which  metal  will  be  separated  on  the  negative  pole,  and,  in 
general,  oxygen  on  the  positive  pole.  As  soon,  however,  as 
the  metal  and  oxygen  have  passed  over  into  the  atomic  condi- 
tion, the  electrolytic  solution  pressure  of  each  comes  into 
action  and  operates  to  drive  them  back  into  the  form  of  ions. 
An  electromotive  force  opposed  to  that  of  the  primary  cur- 
rent is  thereby  set  up.  This  electromotive  force,  which  may 
under  certain  conditions  have  a  tension  higher  than  that  of 
the  primary  current,  is  the  cause  of  a  current  called  the 
polarization  current.  Polarization  must  always  appear  when 
unattacked  electrodes,  as  those  of  platinum,  which  are  exclu- 
sively used  in  quantitative  electrolysis,  are  employed. 


TENSION   AND   ITS   SIGNIFICANCE.  15 

The  tension  required  for  electrolysis  may  always  be  deter- 
mined from  the  consideration  of  the  above  conditions.  It 
must  in  all  cases  be  greater  than  the  resulting  polarization 
current,  for  otherwise,  at  the  commencement  of  decomposition, 
the  electromotive  force  of  the  primary  current  would  be 
counterbalanced  by  the  polarization  tension  and  electrolysis 
would  be  entirely  prevented. 

Le  Blanc,  who  made  a  careful  study  of  the  values  of  the 
tensions  required  for  the  decomposition  of  various  solutions, 
directed  attention  to  the  fact  that  for  the  continued  electrolysis 
of  any  solution  a  definite  minimum  tension,  dependent  directly 
upon  the  polarization  phenomena,  is  required. 

This  so-called  decomposition  tension  value  is  often  given 
as  a  measure  in  quantitative  electrolysis,  and  by  it  is  denoted 
that  electromotive  force  at  which  the  current  is  just  able  to 
pass  through  the  cell.  If  the  electromotive  force  of  the 
primary  current  be  denoted  by  E,  the  current  strength  by 
C,  the  resistance  by  R,  and  the  polarization  tension  by  P, 
then 


E  must  satisfy  this  equation  without  C  being  equal  to   0  ; 
then  only  can  the  current  continuously  decompose  the  solution. 
Le  Blanc  determined  the  following  values  for  the  deeoin- 

,ST 
position  tension  of  y  solutions  : 


ZnSO4  ..............  2.35  volts.  Cd(NO,)a  ............  1.98  volts. 

ZnBra  ..............  1.80     "  CdSO4  ..............  2.03     " 

NiS04  ..............  2.09     "  CdCl,  ..............  1.88    " 

NiCL,  ..............  1.85     "  CoS04  ..............  1.92     " 

Pb(NO,)a  ...........  1.52    "  CoCla  ...............  1.78     " 

AgNO,  .............  0.70     " 


16  QUANTITATIVE   ANALYSIS   BY   ELECTROLYSIS. 


ACIDS. 


Sulphuric  acid 1.67  volts.        Pyroracemic  acid 1.57  volts* 


Nitric  acid 1.69 

Phosphoric  acid.  ...1.70 
Monochloracetic  acid  1.72 
Dichloracetic  acid  ..  1.66 

Malonic  acid 1.69 

Perchloric  acid 1.65 

Dextrotartaric  acid . .  1.62 


Trichloracetic  acid...  1.51 

Hydrochloric  acid 1.31 

Hydrazoic  acid 1.29 

Oxalic  acid 0.95 

Hydrobromic  acid 0.94 

Hydriodic  acid 0.52 


BASES. 


Sodium  hydroxide. . . .  1.69  volts.     —  Methylamine 1.75  volts. 

Potassium  hydroxide. .  1.67     "        —  Diethylamiue 1.68     " 

N 

Ammonium  hydroxide  1.74     "         ^-  Tetramethyl  ammo- 
nium hydroxide..  1.74     " 

The  sulphates  and  nitrates  of  the  alkalies  and  alkaline 
earths  have  all  nearly  the  same  decomposition  tension  value, 
namely,  about  2.20  volts. 

The  values  of  the  decomposition  tension  for  solutions  of 
metallic  salts  are  of  decided  importance  for  quantitative 
electrolysis,  since  by  them  is  given  the  minimum  tension  re- 
quired for  the  precipitation  of  a  metal,  and  also  the  conditions 
under  which  several  metals  may  be  quantitatively  precipitated 
from  the  same  solution  by  simply  altering  the  tension  of 
the  current.  For  example,  zinc  is  not  precipitated  from  a 

IS"  r 

—  ZnSO4  solution  by  a  current  having  a  tension  of  less  than 

N 
2.35  volts,  while  a  —  silver  nitrate  solution  is  decomposed 

at  0.70  volts.  The  silver  may  therefore  be  separated  at  a  ten- 
sion of  less  than  2.35  volts,  the  zinc  remaining  meanwhile  in 
solution.  After  the  separation  of  the  silver,  the  electromotive- 


SIGNIFICANCE   OF   CURRENT   STRENGTH.  17 

force   may  be   increased    to   over    2.35    volts   and    the  zinc 
precipitated  as  metal. 

Kiliani,  whose  early  death  is  to  be  greatly  lamented,  was 
the  first  to  point  out  the  importance  of  the  tension  for  elec- 
trolytic separations.  Somewhat  later,  Freudeiiberg,  basing 
his  work  on  Le  Blanc's  studies,  carried  out,  in  Ostwald's 
laboratory,  a  careful  investigation  of  the  exact  relations.  The 
results  which  he  obtained  will  be  given  in  the  Special  Part,  in 
connection  with  the  discussion  of  the  determination  and 
separation  of  the  respective  metals. 

SIGNIFICANCE    OF    CURRENT    STRENGTH. 

Although  the  choice  of  tension  makes  the  electrolytic 
separation  of  a  metal  possible,  the  condition  of  the  resulting 
precipitate  is  first  of  all  dependent  upon  the  strength  of  the 
current  which  flows  through  the  cell. 

This  follows  from  Faraday's  law,  since  the  number  of 
ions  wThich,  by  discharging  on  the  electrodes,  separate  in  the 
atomic  condition,  in  the  unit  time,  depends  solely  on  the  cur- 
rent strength. 

Irrespective  of  the  tension  employed,  a  current  of  double 
strength  will  precipitate  twice  the  quantity  of  metal  in  the 
same  time.  The  current  strength  therefore  determines  the 
number  of  ions  which  will  discharge  within  a  given  time,  and 
correspondingly  the  rate  of  deposit  on  the  electrode.  With 
relation  to  the  latter,  however,  a  second  factor,  namely,  the 
size  and  si i ape  of  the  electrodes,  is  of  decided  importance, 
since  the  manner  in  which  the  metal  is  deposited  by  a  certain 
current  strength  depends  entirely  on  this.  If  the  area  of  the 
electrode  surface  is  small  and  the  current  density  great,  the 
individual  atoms  of  metal  are  deposited  one  upon  the  other  in 
such  rapid  succession  that  the  precipitate  does  not  adhere 
firmly  to  the  electrode,  but  scales  off.  In  quantitative  elec- 


IS  QUANTITATIVE   ANALYSIS   BY   ELECTROLYSIS. 

trolysis,  the  firm  adherence  of  the  precipitate  to  the  electrode 
is  most  essential  for  the  determination  of  the  weight  of  the 
separated  material. 

When,  on  the  other  hand,  a  very  large  surface  is  offered 
for  the  deposition,  and  a  current  of  low  current  strength  is 
employed,  it  is  impossible  for  a  compact  layer  to  form  and 
the  metal  will  coat  the  surface  in  isolated  patches.  These 
conditions  of  precipitation  are  also  useless  for  quantitative 
determinations. 

The  real  significance  of  the  current  strength  lies  in  its 
ratio  to  the  area  of  the  electrode  surfaces.  This  is  known  as 
the  current  density,  and  as  unit,  a  current  strength  of  one 
ampere  for  100  square  centimeters  electrode  surface,  has  been 
chosen. 

In  this  book,  therefore,  the  current  density  will  always  be 
given  with  reference  to  100  square  centimeters  of  electrode 
surface,  and  will  be  expressed  by  the  symbol  ND]00. 

If,  for  example,  a  current  of  3  amperes  flows  through  a 
cell,  the  electrode  surface  of  which  is  equal  to  250  sq.cm.,  an 
equal  distribution  of  the  lines  of  the  electric  current  being  as- 
sumed, the  current  of  3  amperes  will  be  distributed  over  the 
250  sq.cm.,  and  therefore  every  100  sq.cm.  receives  a  current 

o 

of  —  ampere.    The  current  density,  therefore,  ND100  =  1.2 
2 . 5 

ampere. 

Although  the  current  strength  is  the  same  at  every  point 
in  the  circuit,  the  current  density  of  the  cathode  and  anode 
have  the  same  value  only  when  the  two  electrodes  have 
exactly  equal  dimensions. 

In  the  determination  of  metals  it  is  usually  sufficient  to 
know  the  current  density  at  the  cathode  alone.  On  the  other 
hand,  to  determine  the  halogens,  for  example,  a  knowledge  of 
the  current  density  at  the  anode  is  required. 


SIGNIFICANCE   OF  THE   KESISTANCE.  19 

The  current  strength,  in  the  form  of  the  current  density, 
accordingly  occupies  an  important  place  in  quantitative  elec- 
trolysis. 

SIGNIFICANCE  OF  THE  RESISTANCE. 

The  third  factor  in  Ohm's  Law,  the  resistance,  is  to  be 
considered  chiefly  in  the  selection  of  the  solvent  which  will  he 
most  suitable  for  the  experiment,  and  the  proper  substances 
to  be  added  to  the  electrolyte. 

It  is  evident  that  with  a  certain  given  tension,  which  in 
«very  electrolysis  may  vary  within  certain  limits,  the  speed  of 
the  operation  depends  upon  the  resistance  of  the  solution, 
since  by  this  the  current  strength  is  determined  according  to 
the  equation 

E 


It  is  therefore  necessary  to  have  the  conductivity  of  the 
solution  as  high  as  possible.  Since  aqueous  solutions  are  ex- 
clusively employed  in  quantitative  electrolysis,  this  is  accom- 
plished by  the  addition  of  certain  substances,  the  natures  of 
which  are  dependent  upon  the  chemical  properties  of  the  metals 
in  the  solution.  In  some  cases  acids  are  used  ;  in  others, 
bases  or  salts.  The  proper  substances  can  only  be  determined 
by  experience. 

A  fundamental  requirement  of  the  substance  added  and 
one  which  is  independent  of  the  chemical  properties  of  the 
metal  to  be  precipitated  may,  however,  be  stated.  It  must 
be  a  good  conductor  of  the  current  and  must  form  no  decom- 
position products  which  are  insoluble  or  are  detrimental  to  the 
analysis.  Alkalies  and  acids,  which  after  their  decomposition 
are  again  regenerated  at  the  electrodes,  are  therefore  suitable, 


20  QUANTITATIVE   ANALYSIS    BY    ELECTROLYSIS. 

as  are  also  organic  acids,  the  decomposition  products  of  which 
are  given  off  in  a  gaseous  form.  This  last  condition  is  fulfilled 
by  oxalic  acid,  which,  on  account  of  its  great  solubility,  is  of 
special  importance  in  the  electrolysis  of  metals,  particularly  in 
the  form  of  double  salts. 

[A  few  words  on  the  theory  of  the  conductivity  of  solu- 
tions will  perhaps  be  in  place  here.  Since  the  passage  of 
electricity  through  an  electrolyte  is  always  accompanied  by  the 
transference  of  material,  the  power  which  a  solution  has  for 
conducting  the  electric  current  must  depend  directly  upon  the 
nature  of  the  substances  which  are  held  in  solution.  As  has 
already  been  stated,  the  ions  alone  are  bearers  of  electric 
charges,  the  undissociated  molecules  taking  no  part  in  the 
transportation  of  electricity.  The  conductivity  of  a  solution 
therefore  depends  upon  the  number  of  ions  which  it  con- 
tains, and  upon  the  nature  of  the  ions  themselves.  If,  in  two 
equal  volumes  of  a  solution  of  the  same  substance,  one  contains 
twice  as  many  ions  as  the  other,  the  other  physical  conditions  of 
both  being  the  same,  the  conductivity  of  the  former  will  be 
twice  that  of  the  latter. 

The  ratio  of  the  dissociated  part  to  the  total  amount  of 
substance  present  is  called  the  degree  of  dissociation.  The 
degree  of  dissociation  varies  for  different  substances,  but  in 
all  cases  for  fairly  concentrated  solutions  increases  with 
dilution.  For  very  concentrated  solutions  the  degree  of  dis- 
sociation is  very  low.  Concentrated  sulphuric  acid,  for 
example,  is  practically  a  non-conductor,  although  a  dilute 
solution  of  the  same  possesses  a  relatively  high  conductivity. 

The  ease  with  which  a  current  can  pass  through  a  cell 
containing  two  electrodes  immersed  in  an  electrolyte  depends 
then  upon  the  distance  by  which  the  electrodes  are  separated, 
and  upon  the  number  and  nature  of  the  ions  which  are  be- 
tween them. 


THEORY   OF   ELECTROLYTIC    PRECIPITATION.  21 

The  resistance  of  the  cell  may  be  decreased  in  a  variety 
of  ways.  For  example,  a  substance  possessing  a  high  degree 
of  dissociation  may  be  added  to  the  solution,  and,  provided 
that  it  does  not  materially  influence  the  degree  of  dissociation 
of  the  substance  already  present,  the  number  of  ions  between 
the  electrodes  will  be  accordingly  increased,  and  thereby  the 
conductivity  of  the  solution.  The  solution  may  also  be 
warmed.  The  effect  of  this  is  generally  to  slightly  increase 
the  degree  of  dissociation,  but  more  especially  to  decrease  the 
viscosity  of  the  solvent,  as  a  result  of  which  the  ions  experi- 
ence less  resistance  to  their  movements  through  the  solution, 
and  the  passage  of  the  current  is  thus  expedited.  Another 
means  at  hand  is  to  diminish  the  distance  between,  or  to  in- 
crease the  size  of,  the  electrodes.  The  effect  of  the  former  is 
readily  understood,  and  by  the  latter  a  greater  number  of  ions 
are  brought  within  the  sphere  of  action. 

A  word  more  as  to  the  influence  of  ions  themselves. 
Since  the  electricity  is  transported  by  the  ions,  the  fate  of 
migration  of  the  same  must  be  ar^  important  factor  of  the 
conductivity.  Hydrogen  of  all  ions  has  tke  highest  velocity  of 
migration.  Accordingly  all  highly  dissociated  acids  are  good 
conductors.  The  hydroxyl  ion  comes  next,  which  explains 
the  relatively  high  conductivity  of  the  bases.  The  conduc- 
tivity of  a  solution  is  indeed  nothing  more  than  a  function 
of  the  rates  of  migration  of  the  cathion  and  anion. — Trans.~\ 

THEORY  OF  ELECTROLYTIC  PRECIPITATION. 

When  viewed  from  the  standpoint  of  the  theory  of  elec- 
trolytic dissociation  the  processes  of  quantitative  electrolysis 
may  be  generalized  as  follows.  Quantitative  determinations 
may  be  divided  into  two  classes  according  to  whether  the  de- 
termination of  a  cathion  (metal)  or  an  anion  (halogen  or 


22  QUANTITATIVE  ANALYSIS   BY   ELECTROLYSIS. 

metal  peroxide)  is  concerned.  What  takes  place  in  the  former 
case  is  evident  without  further  explanation.  The  metal  ions. 
migrate  in  the  direction  of  the  positive  current,  to  the 
cathode ;  discharge,  and  separate  on  the  electrode  in  the  form 
of  a  smooth  metallic  coating.  The  halogens  may  be  separated 
in  a  similar  manner ;  but  since  some  of  them  are  gaseous  or 
liquid,  it  is  not  practical  to  weigh  them  directly.  Instead, 
therefore,  of  using  inert  platinum  electrodes  as  are  used  with 
metals,  silver  electrodes  are  employed.  With  these  the 
halogen  atoms  combine,  at  the  moment  of  discharge,  to  form 
halogen  silver  compounds  which  adhere  firmly  to  the  elec- 
trode. The  increase  in  weight  of  the  electrode  gives  directly 
the  quantity  of  the  halogen  which  has  separated. 

The  process  in  the  separation  of  metal  peroxides  (only 
lead  and  manganese  peroxides  will  be  here  considered)  is. 
somewhat  more  complicated.  Formerly  the  formation  of 
PbOa  and  MnO2  was  attributed  to  an  oxidation  brought  about 
by  the  electrolytically  generated  oxygen.  The  investigations, 
of  Liebenow*  and  Lobf  have  made  it  appear  that  lead  peroxide 
ions  and  manganese  peroxide  ions  are  already  present  in  the 
solutions.  Since  the  peroxides  separate  from  strong  nitric 
acid  solutions,  it  must  be  assumed  that  through  the  oxidizing 
power  of  this  acid  oxygen  ions  are  formed  in  the  solutions, 
and  that  these  combine  with  the  lead  or  manganese  ions  to- 
form  peroxide  ions.  Since  in  the  peroxides  of  the  bivalent 
metals  the  two  positive  charges  of  the  metal  are  combined 
with  the  four  negative  charges  of  the  two  oxygen  atoms,  the 
resulting  peroxide  ion  therefore  possesses  two  negative  charges 
and  consequently  behaves  like  a  bivalent  anion.  It  is  precipi- 
tated on  the  positive  electrode  as  a  smooth,  adherent  coating 

*  Zeitschr.  f.  Electrochemie,  1895-96,  pp.  420,  653. 
fZeitschr.  f.  Eleclrocbemie,  1896-97,  p.  100. 


DETERMINATION   OF  THE   CURRENT   MAGNITUDES.    23 

in  a  form  similar  to  that  of  a  metal.  The  details  of  the  reac- 
tions will  be  given  in  the  Special  Part. 

From  what  has  been  said,  the  necessity  of  accurate  data  in 
the  performance  of  electro-analyses  is  obvious,  for  unless  all 
the  important  conditions  are  determined  and  recorded  the  ex- 
periment cannot  be  accurately  repeated. 

Since  the  determination  of  the  resistance  of  the  liquid  in 
the  cell  is  beyond  the  scope  of  analytical  work,  therefore,  in- 
stead of  this,  the  exact  volume  and  composition  of  the  solu- 
tion, as  well  as  the  size  and  shape  of  the  electrodes,  must  be 
stated.  In  addition  to  this  the  tension  at  the  electrodes,  the 
current  strength  as  read  directly  on  the  amperemeter,  and  the 
calculation  of  the  current  density  from  the  current  strength, 
for  the  electrode  on  which  the  quantitative  precipitation  has 
taken  place,  must  be  given.  All  electrical  relations  are  influ- 
enced by  the  temperature,  so  that  an  exact  knowledge  of  this 
is  most  essential.  The  length  of  time  required  for  the  elec- 
trolysis and  the  nature  of  the  source  of  current  having  been 
specified,  all  adequate  and  necessary  data  are  at  hand  to  enable 
every  one  to  repeat  the  analysis  under  exactly  similar  condi- 
tions. 


DETERMINATION  OF  THE  CURRENT 
MAGNITUDES. 

1.  MEASUREMENT  OF  THE  CURRENT  STRENGTH. 

The  current  strength  is  measured  either  by  means  of  the 
chemical  or  the  electromagnetic  action  of  the  current.  The 
chemical  instruments  are  the  oxyhydrogen  gas  voltameter  and 
the  weight  voltameter  ;  the  first  of  which  depends  upon  the 
volume  of  gas  produced,  the  second  upon  the  weight  of 
metal  precipitated. 


QUANTITATIVE  ANALYSIS   BY   ELECTROLYSIS. 


THE   OXYHYDROGEN  GAS   VOLTAMETER. 

The  construction  of  the  apparatus  is  shown  in  Fig.   1. 
The  cylindrical  vessel  g  is  partly  filled  with  pure  dilute  33 


FIG.  1. 

per  cent  sulphuric  acid.  The  platinum  wires  d  and  d'  welded 
to  the  platinum  strips  p  and  p*  are  fused  into  the  walls  of 
the  vessel  g.  This  latter  stands  in  a  large  cylinder  C  of  water 
which  serves  to  cool  it.  The  platinum  wires  end  in  the  screws 

*In  the  apparatus  used  by  the  author,  the  platinum  electrodes  are 
31  X  13  mm.,  aiid  are  distant  from  each  other  20  mm. 


THE   OXYHYDBOGEN    GAS    VOLTAMETER.  25 

s  and  s',  wliich  are  connected  with  the  battery.  The  oxy- 
hydrogen  gas,  as  it  is  formed,  passes  through  the  tube  /•,  which 
contains  a  little  water,  and  is  then  collected  in  the  measuring- 
tube  K,  which  is  graduated  into  -^  cc,  and  filled  with  water. 
To  measure  an  electric  current  with  the  voltameter,  the  water 
over  which  the  gas  is  collected  is  first  saturated  with  oxyhy- 
drogen  gas,  and  then,  by  the  use  of  a  watch  with  second-hand, 
the  volume  of  gas  is  observed  which  the  current  yields  in  a 
minute,  or,  if  the  current  is  weak,  in  a  longer  time.  To 
compare  observations,  the  volume  should  be  reduced  to  0° 
and  760  rnm.  pressure. 

v  =  observed  volume  of  oxy hydrogen  gas ; 
vl  =  normal  volume  (at  0°  and  760  mm.); 
t  —  observed  temperature ; 
h  —  pressure  reckoned  in  mm.  of  mercury. 


1  +  0.00367*  '760* 

Let  I  indicate  the  height  of  the  column  of  liquid,  s  the 
density  of  the  liquid,  and  I  the  barometric  height ;  then 


h  =  b  -  I 


-x- 


13.6* 

The  oxyhydrogen  gas  voltameter,  which  unfortunately  is 
still  frequently  used,  is  quite  unsuited  to  the  purpose  for 
which  it  is  intended,  since,  among  other  disadvantages,  it 
possesses  a  high  tension  which  under  some  circumstances  may 
be  much  greater  than  that  of  the  experiment.  In  addition  to 
this,  the  comparison  of  measurements  made  with  different  oxy- 
hydrogen gas  voltameters  is  only  possible  when  the  instru- 

*  13.6  sp.  g.  of  mercury 

""HF^ 

UNIVERSITY 
^CALIFOR^, 


26  QUANTITATIVE   ANALYSIS   BY    ELECTROLYSIS. 

ments  correspond  absolutely  with  one  another  in  construction, 
a  condition  not  possible  in  practice. 


THE  WEIGHT  VOLTAMETER. 

The  weight  voltameter,  which  is  used  in  the  form  of  a 
copper  or  silver  voltameter,  has 
found  quite  as  little  application  in 
electrolytic  laboratories.  Its  con- 
struction is  given  in  Fig.  2. 

Both  methods  have  been  sup- 
planted by  the  electromagnetic 
measuring  instruments,  some  of 
which  make  use  of  the  deflection  of 
the  magnetic  needle  caused  by  the 
current,  and  others  of  the  magnetic 
properties  induced  in  a  soft  iron 
core.  To  the  former  class  belong  the 
sine  and  tangent  galvanometers ;  to 
the  latter,  the  instrument  most  used 
in  practice,  i.e.,  the  amperemeter. 


FIG.  2. 


THE   TANGENT   GALVANOMETER. 

In  the  tangent  galvanometer  (Fig.  3)  there  is  a  small  mag- 
net which  has  its  plane  of  swing  horizontal  and  at  right  angles 
to  the  plane  of  a  ring  shaped  circuit.  The  actual  position  of 
the  magnet  when  at  rest,  as  well  as  that  of  the  plane  of 
the  windings  of  the  circuit,  is  the  magnetic  meridian. 
When  a  current  flows  through  the  wire  ring,  there  results 
a  deflection  of  the  magnetic  needle,  the  amount  of  which 
depends  upon  the  strength  of  the  current  and  the  number 
of  windings.  If  H  denotes  the  horizontal  component  of 


THE   TANGENT   GALVANOMETER.  27 

the  earth's  magnetic  field,  n  the  number  of  turns  of  wire  in 
the  ring,  r  the  radius  of  the  ring,  and  0  the  angle  of  deflection 


FIG.  3. 


caused  by  the  current,  then  the  current  strength 
C  = H  •  tan0. 


n 


is  a  constant  for  each  instrument,  which  may  be 

determined  by  connecting  it  witli  a  source  of  known  current 
strength,  according  to  the  equation 

r  C 


n       H  •  tan  0 


28  QUANTITATIVE   ANALYSIS   BY    ELECTROLYSIS. 

The  current  strength  to  be  determined  may  then  be  easily 
found ;  it  is  C  =  K  •  H  •  tan  (p. 


THE    SINE    GALVANOMETER. 

The  sine  galvanometer  (Fig.  4)  differs  from  the  tangent 
galvanometer  in  that  the  plane  of  the  windings  is  not  fixed  in 
the  meridian,  but  is  turned  in  the  direction  of  the  needle  dis- 


FIG.  4. 

placed  by  the  current,  until  the  position  of  the  latter  again 
corresponds  with  the  plane  of  the  circuit.  The  angle  through 
which  the  current  circle  has  been  turned  from  its  original 
position  in  the  magnetic  meridian  is  then  read  off.  The  cur- 
rent strength  is 

C  =  K  -  H  -  sin  0, 

where  K  denotes  the  constant  reduction  factor  of  the  instru- 
ment. 


OTHER  FORMS    OF   GALVANOMETERS. 


OTHER    FORMS  OF  GALVANOMETERS. 

The  galvanometers  which  are  used  for  making  the  most 
accurate  measurements  depend  likewise  upon  the  displacement 


FIG.  5. 

of  the  magnetic  needle  by  a  circular  current.  Here,  however, 
the  needle  is  suspended  by  a  fine  cocoon  fiber,  between  spools 
containing  a  very  large  number  of  turns  of  wire.  In  order  to 
remove  the  magnet  from  the  influence  of  the  earth's  magnet- 
ism, a  pair  of  so-called  astatic  needles  are  frequently  em- 


30 


QUANTITATIVE  ANALYSIS   BY    ELECTROLYSIS. 


ployed.  Two  exactly  similar  magnets  are  rigidly  connected 
with  one  another,  so  that  the  north  pole  of  the  one  is  situated 
exactly  over  the  south  pole  of  the  other,  and  correspondingly 
the  south  pole  of  the  first  above  the  north  pole  of  the  second. 
By  this  arrangement  the  effect  of  the  earth's  magnetism  is 
neutralized  by  the  two  needles. 


FIG.  6. 


FIG.  7. 


The  angle  of  displacement  is  read  by  means  of  a  telescope 
and  a  mirror  which  swings  with  the  needle.  The  scale, 
which  is  rigidly  clamped  to  the  telescope,  shows  the  divisions 
which  correspond  to  the  deflection  of  the  needle. 

Figures  5  and  6  show  two  practical  galvanometers. 

The  effect  of  the  magnetism  induced  in  a  soft-iron  core  is 
made  use  of  in  the  two  instruments,  most  useful  for  electrol- 


SPRING   GALVANOMETER — AMPEREMETER.  31 

jsis;  the  Kolilrauscli  spring  galvanometer  and  the  ampere- 
meter (often  called  ammeter). 

THE    SPRING    GALVANOMETER. 

In  this  apparatus  of  Kohlrausch  (Fig.  7),  a  hollow  cylin- 
der of  sheet  iron  is  suspended  within  a  vertical  solenoid 
by  a  spiral  spring.  When  a  current  is  passed  through  the 
instrument,  the  iron  cylinder  is  drawn  down  into  the  solenoid 
until  the  force  of  attraction  is  equalized  by  the  tension  of  the 
spring.  A  small  pointer  attached  to  the  spring  moves  over  a 
scale  which  is  empirically  graduated  and  gives  the  current 
strength  directly  in  amperes. 

THE    AMPEREMETER. 

In  the  amperemeter  (Fig.  8)  the  solenoid  is  usually  placed 
horizontal  and  carries  eccentrically  a  bent  piece  of  thin  sheet 


FIG.  8. 


iron  which  is  provided  with  a  long  pointer.  This  pointer 
moves  over  a  scale.  These  instruments  under  suitable  con- 
ditions are  extraordinarily  sensitive. 


32  QUANTITATIVE   ANALYSIS    BY    ELECTROLYSIS. 


2.  MEASUREMENT  OF  THE  TENSION. 

For  measuring  the  tension  a  great  number  of  instruments 
are  in  use,  the  application  of  which  depends  upon  the  fineness 
of  the  measurement  which  is  to  be  carried  out.  Two  instru- 
ments are  employed  in  electrolysis,  the  voltmeter  and  the 
torsion  galvanometer,  while  for  exact  determinations  of  dif- 
ferences of  potential,  especially  small  ones,  the  Lippmann 
capillary  electrometer  and  the  quadrant  electrometer,  lately  in 
the  form  improved  by  Nernst,  have  been  generally  adopted. 


THE   VOLTMETER. 

Since,  according  to  Ohm's  Law,  E  =  C  •  E,  every  measure- 
ment of  tension  may  be  referred  to  a  measurement  of  the 
current  strength  provided  that  the  resistance  R  remains  con- 
stant. This  is  done  in  the  voltmeter,  which  has  the  external 
form  of  an  amperemeter,  by  giving  the  solenoid  a  very  high 
resistance,  1000  ohms  for  example.  In  this  way,  the  resist- 
ance of  the  connecting  wire  may  be  left  out  of  consideration, 
and  the  fall  in  tension  is  confined  practically  to  the  coils  of 
the  solenoid  alone.  Further,  the  voltmeter  is  always  connected 
on  a  shunt  circuit. 

If  the  resistance  in  this  shunt  is  low,  then  the  greater  part 
of  the  current  passes  through  it,  causing  a  fall  to  take  place 
in  the  tension  at  the  cell  under  measurement.  The  cor- 
responding value  obtained  for  the  tension  will  therefore  be 
too  low. 

If,  however,  the  resistance  of  the  voltmeter  is  very  high,, 
then  the  current  will  pass  through  the  cell  with  almost  un- 
altered tension  and  only  an  extremely  small  fraction  will  go 
through  the  measuring  instrument  itself.  The  scale  is  so 


THE   TORSIOX   GALVONOMETEK.  33 

constructed  with  reference  to  the  resistance  that  the  amperes 

are  converted  directly  into  volts, 

according  to  the  equation,  which 

for  the  solenoid  mentioned  would 

be 

E  =  1000  -C. 

A  most  excellent  form  of 
apparatus  is  the  voltmeter  of 
Weston  (Fig.  9).  This  gives  the 
value  accurately  to  -^  volt  and 

allows  of  approximation  to  Tfg-  volt.  A  mirror,  placed  below 
the  scale  over  which  the  pointer  moves,  prevents  parallax 
in  reading. 

THE   TORSION   GALVANOMETER. 

The  principle  of  this  instrument  is  electromagnetic. 
A  light  bell  magnet  swings  between  two  parallel  and  perpen- 
dicular coils  containing  many  windings  of  wire.  These  two 
spools  are  so  connected  with  one  another  that  the  current 
flowing  through  each  of  them  tends  to  deflect  the  magnetic 
needle  in  the  same  direction.  The  magnet,  the  swinging  of 
which  is  usually  retarded  by  copper  damping,  is  suspended 
from  the  cover  of  the  case  by  a  spiral  spring.  To  this  spring 
a  horizontally  moving  pointer,  which  is  just  beneath  the  glass 
cover  of  the  instrument,  is  attached.  A  second  pointer  is 
fastened  directly  to  the  magnet. 

The  instrument  is  used  as  follows :  By  revolving  the  case 
the  needle  is  brought  into  the  magnetic  meridian,  so  that  the 
two  pointers  correspond  to  the  zero  point  of  the  scale  on  the 
glass  cover.  Care  must  be  taken  that  the  magnet  swings 
entirely  free,  which  is  insured  by  setting  the  instrument 
exactly  horizontal  by  means  of  the  foot-screws.  If  the 


34 


QUANTITATIVE  ANALYSIS   BY   ELECTROLYSIS. 


binding-screws  of  the  galvanometer  are  now  connected  with 
the  source  of  current  or  with  the  cell,  the  tension  of  which  is 
to  be  determined,  the  magnet  will  be  deflected  from  its 
position  of  rest  and  the  pointer  attached  to  the  spiral  spring 
must  be  turned  a  certain  distance  in  order  to  bring  the  pointer 
rigidly  connected  to  the  magnet  back  to  the  zero  point  on 


FIG.  10. 


the  scale,  and  thereby  to  return  the  magnet  itself  to  its  former 
position.  In  this  operation  the  pointer  fastened  to  the  top  of 
the  spiral  spring  has  been  moved  through  a  certain  number 
of  divisions  on  the  scale,  and  this  scale  is  empirically  calibrated, 
so  that  by  the  position  of  the  pointer  the  tension  is  given  in 
_i_  volt.  By  interposing  resistance,  the  sensitiveness  may  be 


THE   LIPPMANN   CAPILLARY   ELECTROMETER.  35 

so  decreased  that  the  divisions  correspond  to  tenths  or  to 
whole  volts.  Fig.  10  shows  a  torsion  galvanometer  of  the 
type  manufactured  by  Siemens  &  Halske  (Berlin). 

The  principle  of  the  construction  of  the  instrument  is 
similar  to  that  of  the  voltmeter.  The  deflection  of  the  needle 
is  of  course  proportional  to  the  current  strength,  but  since  the 
resistance  of  the  spool  windings  is  very  high  and  remains 
constant,  a  strict  proportion  exists  between  the  intensity  and 
the  tension,  so  that  the  direct  reading  in  volts  is  made  possible 
by  the  use  of  the  equation 


THE   LIPPMANN   CAPILLARY   ELECTROMETER. 

This  instrument  is  chiefly  employed  in  the  measurement  of 
electromotive  forces  by  the  Poggendorf  compensation  method, 
most  practical  in  the  arrangement  described  by  Ostwald. 

In  this  a  known  electromotive  force  is  opposed  to  the  one 
which  is  to  be  measured,  and  the  former  is  modified  through 
alterations  of  the  resistance  by  certain  known  amounts,  until 
the  two  electromotive  forces  are  equal  and  compensate  one 
another. 

In  practice,  an  element  of  known  electromotive  force  is 
so  connected  with  a  resistance-box,  which  contains  for 
example  1000  ohms,  that  the  whole  fall  in  potential  takes 
place  through  the  1000  ohms.  Every  resistance  of  10  ohms 
is  provided  with  a  clamp  to  which  a  wire  may  be  connected. 
If  the  known  electromotive  force  is  for  example  1  volt,  this 
will  be  distributed  over  the  resistance  in  such  a  way  that  the 
1000  ohms  will  represent  a  fall  of  1  volt,  every  100  ohms 
one  of  0.1  volt,  and  every  10  ohms  one  of  0.01  volt.  The 
electromotive  force  to  be  measured  is  now  connected  with  the 


36  QUANTITATIVE  ANALYSIS   BY    ELECTROLYSIS. 

two  clamps  of  the  resistance-box  in  such  a  manner  that  it  will 
be  opposed  to  the  known  electromotive  force  and  the  resist- 
ance between  the  two  clamps  is  varied,  until  the  two  electro- 
motive forces  are  equal  and  compensate  one  another.  If,  for 
example,  the  resistance  between  the  clamps  is  100  ohms,  then 
the  electromotive  force  to  be  measured  is  equal  to  0.1  volt,  for 
110  ohms  it  would  be  0.11  volt,  for  120  ohms  0.12  volt,  etc. 
In  order  to  determine  when  the  electromotive  force  which 
is  being  measured  is  equal  to  the  opposed  electromotive  force, 
a  Lippmann  electrometer  of  the  form  given  by  Ostwald  *  is. 
employed.  (Fig.  11.) 


FIG.  11. 

' '  A  platinum  wire,  partly  encased  in  a  glass  capillary, 
leads  from  an  insulated  binding- screw  and  extends  into  the 
mercury  at  the  bottom  of  the  bulb  5,  which  also  contains 
above  the  mercury  a  10  per  cent  sulphuric  acid  solution. 
The  capillary  tube  c  opening  into  the  bulb  ~b  is  filled  in  its 
upper  part  with  acid ;  its  lower  part  contains  mercury,  like- 
wise the  tube  d,  which  is  in  connection  with  a  second  binding- 
screw.  The  position  of  the  mercury  in  the  capillary  tube  o 
may  be  regulated  through  altering  the  inclination  of  the  capil- 
lary by  means  of  the  screw  at  /.  That  this  apparatus  may 

*Ztschr.  f.  pbys.  Chem.,  1890,  p.  471. 


THE   QUADRANT   ELECTROMETER.  37 

give  satisfactory  results  it  should  be  short- circuited  just  before 
use,  and  consequently  it  was  connected  with  a  switch  so  con- 
structed that  on  breaking  the  current  the  electrometer  was  always 
short-circuited,  and  on  making  the  current  this  connection 
within  itself  was  destroyed .  In  measuring  electromotive  forces, 
so  much  of  the  resistance  of  the  box  was  brought  between  the 
movable  clamps  that  the  mercury  remained  at  rest  on  closing 
the  circuit.  A  millimeter  scale  placed  beneath  the  capillary, 
and  a  lens  above  it,  aided  in  the  reading.  It  wras  possible 
to  approximately  estimate  to  a  thousandth  volt.  One  hun- 
dredth volt  corresponded  to  3£  divisions  on  the  scale." 

The  Lippmann  capillary  electrometer,  the  theory  of  which 
cannot  be  entered  upon  here,  depends  upon  the  fact  that  the 
surface  tension  of  mercury  alters  under  varying  electrical  con- 
ditions. When  the  two  opposed  electromotive  forces  are 
equal,  then  the  mercury  is  electrically  neutral  and  the  menis- 
cus returns  to  its  normal  position. 

THE   QUADRANT    ELECTROMETER. 

The  quadrant  electrometer,  which  was  constructed  in  the 
most  varied  forms  by  "W.  Thomson  and  'is  very  generally  used 
for  the  measurement  of  potentials,  has  the  following  general 
construction. 

Four  separated  sectors  of  a  flat,  cylindrical  metal  box  rest 
upon  four  insulating  glass  supports.  Each  of  these  sectors  is 
called  a  quadrant.  Each  pair  of  oppositely  located  quadrants 
is  in  metallic  connection.  Within  the  hollow  space  formed  by 
the  four  quadrants  the  so-called  needle,  a  thin,  horizontal  plate 
of  aluminium,  is  suspended  by  a  fine  wire  which  also  carries 
a  mirror  for  reading.  A  wire  leads  from  the  needle  to  a  vessel 
filled  with  sulphuric  acid,  situated  beneath  the  quadrants. 

In  using  the  instrument,  the  aluminium  needle  is  charged 
to  a  comparatively  high  potential  by  connecting  it  with  some 


38 


QUANTITATIVE   ANALYSIS   BY    ELECTROLYSIS. 


FlG.  12. 


source  of  electricity,  usually  a  Leyden  jar,  through  the  wire 
dipping  into  the  sulphuric  acid.  One  pair 
of  quadrants  is  then  grounded,  and  the 
other  pair  is  connected  to  one  pole  of  the 
electromotive  force  to  be  measured,  the 
second  pole  of  the  electromotive  force  being 
connected  with  the  earth.  So  long  as  all  the 
quadrants  have  the  same  potential,  the 
aluminium  needle  remains  at  rest ;  the 
difference  between  the  potential  of  the  earth 
and  the  one  under  consideration  is  deter- 
mined from  the  deflection  of  the  needle 
measured  by  means  of  the  mirror  and  scale. 
Nernst  and  Dolezalek  *  have  made  the 
employment  of  this  instrument  more  simple 
and  accurate.  They  avoid  the  operation  of 
charging  the  aluminium  needle  before  use, 
by  employing  a  small  perpendicularly  hung 
Zamboni  pile  having  a  tension  of  about 
1400  volts.  (Figs.  12  and  13.) 

A  small  Zamboni  pile  Z,  suspended  by 
the  quartz  fiber  /*,  has  fastened  to  its  two- 
poles  the  electrometer  needles  N,  and  N2, 

which  swing  in  the  quadrant  boxes  Q,  and  Q3,  placed  one 

above  the  other. 

The  measurement  of  a  difference  of  potential  is  carried 

out  by  comparing  the  instrument  with  a  normal  tension,  or  by 

the  compensation  method. 

SOURCES   OF  CURRENT. 

Two  classes  of  current  supply  are  employed  in  electrolysis,, 
chemical  and  physical.      The  first  class  is  represented  by  the 

*Ztschr.  f.  Electrochemie,  1896-97,  p.  1. 


FIG.  13. 


LECLANCHE   CELL. 


39 


galvanic  elements,  which  are  further  divided  into  primary 
and  secondary  elements,  according  as  the  difference  of  poten- 
tial is  directly  due  to  a  chemical  reaction  or  to  a  polarization 
current  (accumulators). 

To  the  second  class  belong  the  electromagnetic  machines 
and  thermopiles.  The  most  important  apparatus  will  be 
briefly  described  in  the  following  section. 

1.     PRIMARY   GALVANIC   ELEMENTS. 

LECLANCHE     CELL. 

This  is  a  one-fluid  cell  using  a  solution  of  ammonium 
chloride,  which  surrounds  the  negative  pole,  the  zinc.  The 


FIG.  14. 


cell  is  much  used  in  the  form  shown  in  Fig.  14:.     In  the  jar, 
which  is  square   in   section,  with  a  rounded   projection  at 


40 


QUANTITATIVE   ANALYSIS    BY    ELECTROLYSIS. 


one  corner,  stands  a  porous  clay  cup,  from  which  projects  a 
block  of  carbon  K  surrounded  by  coarsely  pulverized  man- 
ganese dioxide,  or  a  mixture  of  manganese  dioxide  and  retort 
carbon.  In  the  projecting  rounded  corner  is  a  stout  rod  Z 
of  amalgamated  zinc.  The  carbon  and  zinc  are  both  provided 
with  binding  screws,  and  are  immersed  in  a  concentrated 
solution  of  ammonium  chloride. 

Leclanche*   also   uses,   in   place    of   the   powdered   man- 
ganese dioxide,  compressed  prisms   (shown  in  Fig.  15)  con- 


FIG.  15. 


sisting  of  40  parts  manganese  dioxide,  55  parts  gas  carbon, 
arid  5  parts  shellac ;  a  little  potassium  sulphate  is  also 
added  to  increase  the  conductivity.  The  porous  cup  is 
thus  dispensed  with.  This  cell  has  an  electromotive  force  of 
1.48  volts. 


MEIDINGER   CELL. 


41 


MEIDINGER    CELL. 

In  contrast  to  the  Leclanche*  cell,  that  of  Meidinger  con- 
tains two  liquids,  solutions  of  magnesium  and  copper  sul- 
phates. The  element  is  constructed  as  follows :  In  the  glass 
vessel  G  (Fig.  16)  stands  a  smaller  glass  #,  and  in  this  a 
copper  cylinder  K  to  which  an  insulated  copper  wire  D  is 
fastened. 

A  second  cylinder  Z  of  zinc,  to  which  the  projecting  wire 
D1  is  fastened,  is  placed  in  the  upper  part  of  the  vessel  G. 

The  balloon-shaped  glass 
B,  filled  with  crystals  of 
copper  sulphate,  closes  the 
cell.  The  cell  is  filled  to 
about  three-fourths  of  its 
capacity  writh  a  solution 
of  1  part  crystallized  mag- 
nesium sulphate  in  7  parts 
of  water ;  and  the  balloon- 
shaped  flask  containing 
copper  sulphate  is  filled 
up  with  water,  closed  with 
a  stopper  fitted  with  the 
glass  tube  r,  and,  as  the 
FIG.  16.  cut  shows,  inverted  in 

the    cell. 
Electromotive  force  about  1  volt. 


42  QUANTITATIVE  ANALYSIS   BY   ELECTROLYSIS. 

DANIELL    CELL. 


In  a  jar  of  glass  (Fig.  IT)  is  a  porous  clay  cup  T,  and  in 
this  a  cylinder  of  cast  zinc,  the  negative  pole  (Fig.  18).    The 


FIG.  17.  FIG.  18. 

porous  cup  is  surrounded  by  a  cylinder  of  sheet-copper  K> 
the  positive  pole. 

The  cylinder  of  amalgamated  zinc  *  stands  in  dilute  sul- 

*  The  zinc  is  easily  amalgamated  by  plunging  it  into  mercury,  on  the 
surface  of  which  a  little  hydrochloric  acid  has  been  poured.  The  amalga- 
mated cylinder  is  then  placed  in  a  vessel  of  water  to  remove  the  hydrochloric 
acid,  and  allow  the  excess  of  mercury  to  drop  off. 


DANIELL   CELL.  43 

plmric  acid  (1 :  20),  and  the  copper  cylinder  in  a  solution  of 
copper  sulphate  ;  the  sulphuric  acid  may  be  replaced  by  a 
solution  of  zinc  sulphate. 

The  element  has  an  electromotive  force  of  1.079  volts. 

[The  modification  of  the  Daniell  cell  known  as  the  gravity 
cell  is  the  form  commonly  in  use  for  telegraph  batteries  in 
this  country,  and  is  the  cheapest  and  most  convenient  cell 
for  constant  batteries  to  yield  currents  of  moderate  strength 
in  scientific  laboratories.  It  is  very  generally  thus  used. 
The  copper  is  placed  at  the  bottom  of  the  jar ;  an  insulated 
copper  wire  is  riveted  to  it,  long  enough  to  pass  up  through 
the  solutions  and  connect  with  a  binding  screw  on  the  zinc 
of  an  adjacent  cell,  or  with  the  wire  which  serves  to  conduct 
the  current  to  the  solution  for  electrolysis.  The  bottom  of 
the  jar,  about  the  copper,  is  filled  with  copper  sulphate ;  the 
zinc,  a  heavy  casting  with  large  surface,  is  suspended  a  few 
inches  below  the  top  ;  and  the  jar  is  filled  with  water  some- 
times acidulated  with  sulphuric  acid.  After  standing  a  few 
hours,  the  copper 'sulphate  dissolves  ;  copper  is  precipitated, 
and  zinc  dissolved ;  and  the  jar,  in  its  normal  working  state, 
thus  contains  two  solutions ;  the  heavier,  of  copper  sulphate, 
below,  and  the  lighter,  of  zinc  sulphate,  above.  The  porous 
cup  of  the  Daniell  cell  is  thus  dispensed  with,  and  the  zinc 
does  not  require  amalgamation. 

The  cut  (Fig.  19)  shows  one  of  the  simplest  gravity  cells, 
having  the  zinc  in  the  so-called  "  crow-foot "  shape,  hanging 
directly  on  the  edge  of  the  jar,  and  furnished  with  a  binding- 
screw. 

The  outfit  of  the  chemical  laboratory  of  the  Pennsylvania 
State  College,  while  under  the  translator's  charge,  was  found 


44 


QUANTITATIVE  ANALYSIS    BY   ELECTROLYSIS. 


convenient,  and  sufficient  for  the  needs  of  an  ordinary  labora- 
tory for  instruction.  Some  twenty  "crow-foot"  gravity  cells 
were  kept  in  working  condition,  and  eight  Grove  cells  could 
be  set  up  if  needed  for  a  strong  current.  Four  sets  of  con- 
necting-wires were  run  from  the  battery-room  to  the  labora- 
tory desk  set  apart  for  electrolytic  work,  each  set  being  so 
arranged  with  binding-screws  as  to  be  quickly  connected  with 
any  desired  number  of  cells.  (See  under  "  Secondary  Bat- 
teries," p.  47.)  —  Trans.'] 


FIG.  19. 


GROVE     CELL. 

The  positive  pole  is  a  sheet  of  platinum  foil  of  the  form 
shown  in  Fig.  20  ;  this  is  placed  in  a  porous  cup  filled  with 
nitric  acid.  The  negative  pole  is  a  cylinder  of  amalgamated 
zinc  placed  in  a  glass  jar  containing  dilute  sulphuric  acid 
(1 :  20).  Fig.  21  shows  the  arrangement  of  the  cell. 
Electromotive  force  1.81  volts. 


BUNSEN   CELL. 


45 


FIG.  20. 


FIG.  21. 


BUNSEN   CELL. 


In  the  Bunsen  cell,  the  platinum  is  replaced  by  a  prism 
of  retort  carbon  (Fig.  22)  standing  in  a  porous  cup  filled  with 
nitric  acid.  The  negative  electrode,  as  in  the  Grove  cell,  is 
a  cylinder  of  amalgamated  zinc  placed  in  a  glass  jar  filled 
with  dilute  sulphuric  acid  (1 :  20).  The  screw-clamp  shown 
in  Fig.  23  is  often  used  to  fasten  a  metallic  connection  to  the 
carbon  prism.  It  has,  however,  the  disadvantage  that  the 
clamp  is  quickly  oxidized  by  the  decomposition  products  of  the 
nitric  acid,  and  the  contact  thus  broken.  It  is  better,  there- 
fore, to  insert  in  the  carbon  a  metallic  socket  (Fig.  24),  the 
stem  of  which  is  closely  covered  with  platinum  foil. 


46 


QUANTITATIVE   ANALYSIS   BY    ELECTROLYSIS. 


Fig.  25  shows  the  Bimsen  cell  in  its  most  common  form. 
Electromotive  force  1.80  volts. 


FIG.  22. 


FIG.  23. 


FIG.  24. 


FIG.  25. 


CUPRON  ELEMENT. 

A  copper  oxide  plate  and  a  zinc  plate  dip  into  an  aqueous 
sodium  hydroxide  solution,  as  shown  in  Fig.  26.     When  a 

current  is  produced  the  following  re  - 
actions  take  place : 


II.    CuO  +  2H  =  Cu  +  H2O. 

The  current  ceases  as  soon  as  all 
5  the  copper  oxide  is  reduced  or  all  the 
zinc  is  dissolved.      In  the  type  intro- 
FIG.  26.  duced  on  the  market  by  TJmbreit  & 

Matthews  (Leipzig),  when  the  copper  oxide  plate  has  been 


FIG.  27. 


GALVANIC   SECONDARY   ELEMENTS.  47 

reduced  it  may  be  reconverted  into  copper  oxide  by  allowing 
the  plate  to  stand  for  15  hours  in  a  warm  place. 

The  element  has  an  electromotive  force  of  0.8  volt,  with 
an  internal  resistance  of  0.05  ohm. 

[EDISON-LALANDE  ELEMENT. 

This  element,  having  a  form  differing 
somewhat  from  the  above  cuprori  element, 
but  depending  for  the  production  of  the 
current  upon  a  similar  chemical  reaction, 
has  come  largely  into  use  in  the  United 
States. 

Elements  of  this  type,  with  capacities  of 
from  50  to  600  ampere-hours,  are  manufactured,  and  furnish 
a  very  convenient  primary  source  of  electricity.     (Fig.  27.)] 

GALVANIC  SECONDARY  ELEMENTS. 

(ACCUMULATORS,    OR   STORAGE   BATTERIES.) 

While  the  primary  elements  previously  described  furnish 
electrical  energy  through  chemical  reactions  which  involve  a 
gradual  using  up  of  their  component  parts,  the  characteristic 
of  accumulators  lies  in  the  fact  that  by  passing  an  electric 
current  through  them  they  are  brought  into  a  condition  which 
makes  it  possible  for  them  to  furnish  a  polarization  current, 
and  thereby  to  return  to  their  original  condition.  Elements 
of  this  class  are  called  reversible  elements. 

Accumulators  are  therefore  instruments  which  alternately 
convert  chemical  energy  into  electrical  energy,  and  electrical 
energy  into  chemical  energy.* 

The  principle  of  their  construction  depends  upon  the 
behavior  of  lead  plates  in  dilute  sulphuric  acid  on  the  passage 
of  a  current.  If  we  have  two  such  plates  dipped  in  sul- 


*  Elbs,  Die  Akkumulatoren,  1896. 


48 


QUANTITATIVE   ANALYSIS    BY    ELECTROLYSIS. 


phuric  acid,  one  serving  as  cathode,  the  other  as  anode,  it 
will  be  observed,  upon  closing  the  current,  that  a  brown  coat- 
ing forms  upon  the  positive  electrode,  spongy  lead  separating 
out  at  the  same  time  on  the  negative  plate.  This  phenome- 
non is  due  to  the  formation,  at  both  poles,  of  a  saturated  solu- 
tion of  lead  sulphate,  the  lead  ions  of  which  discharge  upon  the 
cathode  as  spongy  lead,  while  at  the  anode  the  lead  and  oxygen 
ions  separate  in  the  form  of  lead  peroxide.  If  the  primary 
current  be  broken  and  the  polarization  current  allowed  to 
discharge  by  establishing  metallic  connection  between  the  two 
plates,  the  following  reaction  takes  place :  The  sulphuric  acid 
dissolves  the  spongy  lead  at  the  negative  pole ;  the  hydrogen 
ions  thus  made  available  migrate  to  the  anode  with  their  posi- 
tive charges  and  reduce  the  lead  peroxide  there  present  to 
lead  oxide,  which  again  forms  lead  sulphate  with  the  sulphuric 
acid.  As  soon  as  all  the  spongy  lead  is  dissolved  and  all  the 
lead  peroxide  is  reduced,  the  polarization  current  ceases,  the 
accumulator  is  discharged,  and  the  original  condition  is  again 
reproduced.  By  means  of  a  new  primary  current  (charging 
current)  the  accumulator  may  be  again  brought  into  an  avail- 
able condition. 


Jar. 


Anode. 
Anode  Plates. 

Cathode  Plates. 
Cathode. 


FIG.  28. 


In  the  construction  of  accumulators,  the  longest  possible 
continuation  of  the  polarization  current  is  aimed  at,  together 
with  the  lowest  possible  internal  resistance  of  the  element. 
The  electromotive  force  between  lead  and  lead  peroxide  in 
dilute  sulphuric  acid  is  approximately  2  volts.  In  order  to 


GALVANIC    SECONDARY   ELEMENTS. 


49 


arrive  at  the  most  practical  construction,  a  number  of  parallel 
plates  are  metallically  connected  together  and  hung  as  cathodes 
in  a  trough  containing  sulphuric  acid.  Similarly,  an  equal 
number  of  anode  plates  are  hung  in  and  so  arranged  that  each 
cathode  plate  is  between  two  anode  plates  and  vice  versa,  each 
anode  between  two  cathode  plates.  (Fig.  28.  View  from 
above.) 

The  time  required  for  charging  depends  upon  the  area  of 
the  plates  and  the  condition  of  the  spongy  lead  and  lead  per- 
oxide. In  order  to  satisfy  all  requirements  the  so-called 


PIG.  29. 


FIG.  30. 


"  active  material  "  (i.e.,  the  spongy  lead  and  lead  peroxide)  is 
attached  to  a  solid  lead  frame  by  the  employment  of  a  series  of 
processes  (''forming,"  etc.),  which 
cannot  be  discussed  here.  Fig.  29 
shows  a  negative,  Fig.  30  a  positive 
plate,  while  Fig.  31  gives  an  accumu- 
lator of  the  form  commonly  used. 

The  first  experiments  with  accumu- 
lators for  the  purposes  of  quantitative 
analysis  were  conducted  in  the  Aachen 
laboratory,  with  apparatus  especially 
constructed  by  Professors  Farbaky  and 
Scheneck  *  in  Schemnitz  (Hungary).  Fio.  81. 

*  Compare  Ueber  die  elektrischeu   Akkumulatoren  von  Farbaky  und 


50  QUANTITATIVE  ANALYSIS   BY   ELECTROLYSIS. 

These  gentlemen  had  the  kindness  to  place  two  accumu- 
lators at  the  author's  service  for  testing,  and  he  made  his 
first  experiments  in  1888,  working  with  R.  Schelle,  at  that 
time  professor  in  the  Schemnitz  Royal  School  of  Mines. 
These  accumulators  have  6  negative  and  5  positive  lead-plate 
electrodes,  each  6  mm.  thick.  The  weight  of  the  electrodes 
is  15.5  kg.,  the  volume  of  the  33$  sulphuric  acid  3.5  1.,  the 
total  weight  of  each  cell  35  kg.  The  active  surface  of  the 
electrodes  is  3133  sq.  cm.,  so  that  the  internal  resistance  is 
very  low,  measuring  between  0.0166  and  0.017  ohm.  The 
accumulators  can  be  charged  at  a  20  to  25  ampere  rate,  and 
yield  in  discharge  at  23,  30,  40,  and  60  ampere  rates  respec- 
tively 150,  148,  140,  and  125  ampere-hours,  with  a  fall  of 
not  over  10$  in  the  voltage.  If  the  discharge  is  lighter,  and 
the  fall  in  the  electromotive  force  less  than  for  lighting  pur- 
poses, as  in  electrolytic  analyses,  an  accumulator  may  yield 
over  250  ampere-hours. 

Two  such  accumulators  were  fully  charged,  until  OH  gas 
was  obviously  disengaged,  by  a  current  of  20  to  25  amperes 
from  the  dynamo.  The  current  was  measured  by  a  Kohlrausch 
galvanometer,  made  by  Hartmann  &  Braun,  Bockenheim, 
Frankfurt  a.  M. ,  the  scale  of  which  read  from  0  to  60  amperes. 
A  second  Kohlrausch  amperemeter,  divided  from  0  to  15  am- 
peres, was  used  to  measure  the  current  taken  from  the  accu- 
mulators for  the  analyses. 

A  Siemens  torsion  galvanometer  showed  a  tension,  for  each 
charged  accumulator,  of  2.05  volts. 

By  the  use  of  these  two  accumulators,  four  to  eight  analyses 
were  carried  on  simultaneously,  and  the  accumulators  kept  in 


Scheneck  (Dingier  polyt.  Journ.,  257,  357);  further,  Bericht  tiber  die  Ak- 
kumulatoren  von  Farbaky  und  Scheneck  von  A.  v.  Waltenhofen.  Zeitschr. 
f.  Elektrotechnik,  1886. 


GALVANIC   SECONDARY   ELEMENTS.  51 

constant  use  day  and  night,   except  for  the  short  intervals 
needed  to  change  the  solutions  for  analysis. 

The  results  of  analyses  extending  over  a  period  of  six  days 
are  subjoined. 

FIRST  DAY. 

Tension  2.55  volts. 

Determination  of  Copper  from  Nitric-acid  Solution. 

Taken  CuSO4)5H2O.  Found  Cu. 

4.0140    g  1.0170     g    =  25.33^ 

4.1376    "  1.0480     "     =  25.33 

2.2340    "  0.5661    "     =  25.34 

2.3575    "  0.5978    "     =  25.35 


Tin  from  the  Acid  Ammonium  Double  Oxalate.* 

Taken  8nCl42NH4Cl.  Found  Sn. 

1.8450  g.  0.5964    g.    =32.33 

2.0210    «  0.6548      "    =32.39 

Antimony  from  Solution  in  Sodium  Sulphide. f 

Taken  Sb2S3.  Found  Sb. 

0.2404  g.  0.1720    g.    =  71.50 

0.2551    "  0.1827     «     =  *1.60 


*  Classen's  method  :  see  Tin. 

t         "  ""...    "  Antimony. 


52  QUANTITATIVE  ANALYSIS   BY   ELECTROLYSIS. 

SECOND  DAY. 
Tension  1.95  volts. 


2.0490   g. 

NiS04+(NH4)2S04,6H20      gave   0.3053    g. 

Ni  =  14.90^* 

2.0180    " 

"      0.3000    " 

"  =14.91 

2.3400    •• 

CoSO4  +  K2SO4,6H2O               "      0.3440    " 

Co  =  14.70f 

2.1200    " 

"      0.3120    " 

"  =14.71 

1.8920    " 

FeSO4  +(NH4)2SO4,6H2O          "      0.2697    " 

Fe  =  14.25^ 

2.1240    " 

"      0.3027    " 

"  =14.25 

1.0 

CuSO4,5H2O                                "      0.2533    " 

Cu  =  25.33^ 

1.0 

"      0.2533    " 

"  =25.33 

1.0 

"      02534    " 

"  =25.34 

1.0 

"      0.2537    " 

"  =25.37 

1.9210    " 

SnCl4  +  2NH4Cl                         "      0.6219    " 

Sn  =  32.371 

2.1320    " 

"      0.6900    " 

"  =32.36 

THIRD  DAY. 

Tension  1.95  volts. 

(Six  simultaneous  analyses.) 

1.0050  g. 

CuS04,5H2O                            gave    0.2550  g. 

Cu  =  25.37£ 

1.0170    " 

"      0.2580    " 

"  =25.36 

1.0006    " 

"      0.2539    " 

"  =25.37 

1.0013    " 

"      0.2540    " 

"  =25.37 

1.5680    " 

SnCl4+2NH4Cl                         "      0.5070    " 

Sn  =  32.34 

2.4520    " 

"      0.7946    " 

"  =  32.40 

*  Classen's  method :  see  Nickel. 

f  "         "Cobalt. 

\         "  "          "  Iron. 

§  From  the  acid  double  oxalate,  Classen's  method. 

|  From  the  acid  ammonium  double  oxalate. 


GALVANIC   SECONDARY   ELEMENTS. 

FOURTH  DAY. 


53 


1.0 

1.0 

1.0 

1.0 

1.0 

1.0 

1.0 

1.0 

2.20 

2.45 

2.1340 

2.4350 


g.  CuSO4,5HaO 


Tension  1.95  volts. 


gave 


NiS04-KNH4)2S04,6H20 

<  <  1 1 

CoSO4  +K2SO4,6H2O 


0.2532 
0.2535 
0.2532 
0.2536 
0.2535 
0.2538 
0.2539 
0.2537 
0.3277 
0.3650 
0.3148 
0.3587 


g.  Cu  = 


Ni 
Co 


25.32? 
25.35 
25.32 
25.36 
25.35 
25.38 
25.39 
25.37 
1489 
14.89 
14.75 
14.73 


FIFTH  DAY. 


Tension  1.95  volts. 


1.0         g.   CuS04,5H20 


1.0 

2.4120 

2.2130 


FeSO4-f(NH4)2S04,6H20 


gave  0.2537  g.  Cu  =  25.37^ 

"      0.2537  "     "  =25.37 

"      0.3438  "    Fe  =  14.25 

"      0.3156  "     "  =14.26 


SIXTH  DAY. 


Tension  1.92  volts. 
(Eight  simultaneous  copper  determinations.) 

1.0  g.   CuSO4,5H2O    gave  0.2533    g.  Cu  =  25.33$ 

1.0  «  " 

1.0  "  « 

1.0  "  " 

1.0  "  « 

1.0  «  " 

1.0  «  « 

1.0  "  « 


(( 

0.2534 

" 

" 

=  25.34 

(« 

0.2536 

tt 

<t 

=  25.36 

ti 

0.2533 

tt 

tt 

=  25.33 

tt 

0.2537 

tt 

« 

=  25.37 

tt 

0.2534 

tt 

tt 

=  25.34 

tt 

0.2536 

tt 

n 

=  25.36 

tt 

0.2535 

« 

tt 

=  25.35 

54  QUANTITATIVE  ANALYSIS   BY   ELECTROLYSIS. 

Fifty  determinations,  it  is  seen,  were  made  in  six  days. 
At  the  end  of  the  sixth  day  the  tension  had  fallen  to  1.85 
volts ;  the  accumulators  were  therefore  fully  charged  by  a 
10-ampere  current,  receiving  an  addition  of  5tt  ampere-hours. 
Since  the  total  capacity  of  such  an  accumulator  is  over  250 
ampere-hours,  it  is  safe  to  assume  that  60  to  TO  determina- 
tions may  be  made  from  one  charge ;  and  the  experience  of 
years  confirms  this  assumption. 

To  ascertain  whether  accumulators  in  use  still  retain  electric 
energy,  their  tension  may  be  measured,  or  the  specific  gravity 
of  the  sulphuric  acid  in  the  accumulator  may  be  determined, 
as  this  is  higher  when  the  accumulator  is  charged. 

Still  another  advantage  in  the  use  of  accumulators  is  found 
in  the  fine  quality  of  the  precipitated  metal,  resulting  from  the 
constancy  of  the  current,  which  much  exceeds  that  from  a 
primary  battery  or  a  dynamo.  In  the  Aachen  laboratory  four 
pairs  of  accumulators  have  been  constantly  in  use  since  1888, 
without  need  of  repair.  Four  accumulators  are  used  in  the 
analytical  laboratory,  for  which  they  have  proved  entirely 
sufficient,  and  four  in  the  author's  private  laboratory. 

GENERAL  RULES  FOR  THE  HANDLING  OP 
ACCUMULATORS . 

The  accumulators  which  are  employed  for  the  purpose  of 
quantitative  analysis  are  not  shipped  from  the  factory  ready 
mounted  for  use,  as  in  the  case  of  the  smaller  types  of  bat- 
teries, but  the  glass  jars,  cathode  and  anode  plates,  and  the 
acid  are  packed  separately.  After  the  glass  jars  have  been 
carefully  cleaned,  the  plates  are  set  in  and  the  jars  are  filled 
with  pure  dilute  acid  (sp.  g.  1.15),  so  that  the  plates  are  com- 
pletely covered  by  the  liquid.  The  density  of  the  acid  is  not 
so  important  as  its  purity.  Either  absolutely  pure  (chlorine- 
free)  acid  must  be  obtained  direct  from  the  manufactory,  or 


RULES   FOR  THE   HANDLING   OF   ACCUMULATORS.      55 

must  be  prepared  by  passing  hydrogen  sulphide  gas  for  several 
minutes  into  the  "  commercially  pure  "  acid.  By  this  opera- 
tion the  traces  of  metals  in  solution,  which  would  otherwise 
prove  extremely  detrimental  to  the  accumulators,  will  be  pre- 
cipitated. After  the  precipitate  has  subsided  and  the  acid 
has  been  decanted,  the  dissolved  hydrogen  sulphide  is  removed 
by  warming  the  acid  for  a  short  time  or  by  blowing  in  air. 

As  soon  as  the  accumulators  have  been  set  up  they  are 
ready  for  charging.  This  is  done  from  a  suitable  source  of 
current  according  to  the  number  of  accumulators  at  hand. 

The  charging  current  should  under  no  circumstances  be 
allowed  to  exceed  the  one  given  for  the  particular  model  by 
the  manufactory.  It  must  therefore  always  be  controlled  by 
measuring  instruments.  The  current  from  a  dynamo  or 
thermopile  may  be  most  practically  used  for  charging,  but  in 
case  of  necessity  Bunsen  or  the  so-called  i  i  gravity  batteries ' ' 
may  be  employed.  The  peroxide  plates  are  connected  with 
the  positive,  the  lead  plates  with  the  negative  pole,  of  the 
charging  current. 

If  a  number  of  accumulators  are  to  be  charged,  they  are 
connected  in  series ;  the  charging  current  must  therefore  have 
an  electromotive  force  proportional  to  the  number  of 
accumulators. 

The  so-called  "capacity"  of  the  accumulator  determines 
the  period  required  for  charging.  By  this  is  understood  its 
production  in  ampere-hours.  For  example,  an  accumulator 
of  100  ampere-hours  capacity,  with  a  maximum  rate  of 
discharge  of  10  amperes,  may  be  discharged  at  a  rate  of 

10  amp.          for  10  hours, 

or    5      "  "  20      "    , 

or    1      "  "  100      "    ,  etc.* 

*  Anleitung  zu  elektrochemischen  Versucheu  vou  Dr.  Felix  Oettel, 
1894. 


£6          QUANTITATIVE  ANALYSIS   BY   ELECTROLYSIS. 

The  period  of  charge  is  therefore  given  by  the  current 
strength  of  the  charging  current  when  the  capacity  is  known. 
As  already  stated,  accumulators  are  reversible  elements,  and 
therefore  the  same  number  of  ampere-hours  must  be  expended 
in  charging  them  as  they  themselves  are  able  to  furnish  when 
fully  charged.  The  termination  of  the  charging  must  be 
determined  either  by  a  measurement  of  the  tension,  since 
every  accumulator  when  fully  charged  hag  a  tension  of  about 
2.2  volts,  or  the  appearance  of  the  formation  of  gas  (so-called 
4 '  boiling ' '  of  the  acid)  may  be  taken  as  the  point  for  stopping 
the  primary  current.  The  appearance  of  hydrogen  shows 
that  the  gas  is  no  longer  used  up  to  reduce  the  lead  sulphate 
at  the  negative  pole,  and  that  the  reaction  upon  which  the 
action  of  the  accumulator  depends  is  completed. 

When  the  batteries  are  charged  for  the  first  time  it  is 
found  best  to  continue  the  supercharging  (generation  of  gas) 
for  one  hour  or  over. 

On  discharging,  the  tension  of  each  accumulator  falls 
rapidly  to  2  volts  and  there  remains  constant.  As  soon  as  the 
tension  falls  below  2  volts  the  batteries  must  be  recharged. 

The  manner  and  method  of  connecting  up  the  batteries  on 
charging  depends  upon  the  nature  of  the  source  of  current. 
If  a  dynamo  is  at  disposal,  the  accumulators  are  so  coupled  in 
series  that  the  tension  of  the  dynamo  exceeds  by  a  small 
amount  the  opposed  tension  of  the  accumulators  (2  volts 
each).  If  the  electromotive  force  of  the  primary  current  is 
too  high,  then  it  must  be  reduced,  either  by  introducing 
resistance  or  by  the  employment  of  a  transformer. 

If  a  Giilcher  thermopile  having  a  tension  of  4  volts  is 
used  for  charging  the  batteries,  the  cells  must  be  differently 
connected.  They  are  then  arranged  in  parallel,  i.e.,  the 
similar  poles  are  connected  with  one  another,  so  that  the 
whole  system  acts  like  a  single  accumulator  with  a  tension  of 


ETJLES  FOR  THE  HANDLING  OF  ACCUMULATORS.       57 

2  volts.  Owing  to  the  small  number  of  amperes  furnished 
by  a  thermopile,  a  considerable  time  is  required  for  charging 
(8  cells  of  8  ampere-hours'  capacity  each  require  32  hours). 

To  insure  the  durability  of  the  accumulators,  the  follow- 
ing rules  must  be  observed :  * 

1.  They  must  be  protected  from  short  circuit. 

2.  The  maximum  rate  of  discharge  given  by  the  manu- 
facturers must  not  be  exceeded. 

3.  Each  element  must  not  be  discharged  below  1.85  volts. 

4.  The  elements  must  not  be  allowed  to  stand  in  an  un- 
charged condition ;  also  when  not  in  use  they  must  be  charged 
once  every  3  or  4  months. 

5.  Violent  shaking  must  be  avoided,  since  it  is  apt  to  cause 
the  falling  out  of  the  active  material. 

[The  Electric  Storage  Battery  Co.  of  Philadelphia,  Pa., 
manufacture  a  number  of  types  of  accumulators  which  are 
particularly  suited  fur  electrolytic  work.  Fig.  32  shows  a 
cell  of  the  so-called  type  E,  a  form  of  cell  especially  suited  for 
small  storage  plants.  The  plates  are  provided  with  very  long 
lugs  which  allow  the  connections  to  be  made  at  such  distance 
from  the  acid  that  the  possibility  of  corrosion  is  entirely 
removed.  The  plates  themselves  are  prepared  by  the  so- 
called  ' '  chloride  process, ' '  which  gives  to  them  great  durability. 
In  employing  this  type  of  battery  in  the  laboratory,  it  is  best 
to  seal  the  jars  with  paraffine.  This  is  done  by  pouring 
melted  paraffine  over  the  acid  in  the  jar.  A  good-sized  rubber 
stopper  is  held  with  its  lower  end  just  touching  the  surface  of 
the  acid,  and  the  melted  paraffine  is  allowed  to  flow  around  it. 
When  the  paraffine  has  completely  solidified,  the  stopper  is 
withdrawn  or  may  be  allowed  to  remain  loosely  in  the  orifice. 


*  Of.  Anleitimg  zu  elektrochemischen  Versuckeii  voii  Dr.  Felix  Oettel, 
1894. 


58 


QUANTITATIVE  ANALYSIS   BY   ELECTROLYSIS. 


Spattering  and  evaporation  are  thus  prevented  and  the  cell 
may  be  moved  about  without  danger  of  slopping. 

The  same  Company  also  make  a  portable  cell  (Fig.  33). 
The  accumulators  are  put  up  in  sealed  rubber  jars,  enclosed 


FIG.  32. 

in  neat  hard -wood  cases  provided  with  handles  and  binding- 
posts.  Various  capacities  are  furnished.  These  cells  are 
especially  convenient  for  small  laboratories  or  others  which 
possess  no  plant  suitable  for  charging.  In  such  cases  the 


RULES    FOR   THE   HANDLING   OF   ACCUMULATORS.      59 

portable  accumulators  may  be  charged  outside  of  the  building 
at  some  power  station  at  a  reasonable  figure.     With  four  such 


FIG.  33. 

cells  of,  say,  100  ampere-hours  capacity  each,  and  the  simple 
arrangement  described  on  page  108,  any  of  the  analyses 
described  in  this  book  may  be  carried  out. 

The  translators  are  indebted  to  the  Electric  Storage 
Battery  Company  for  their  kindness  in  furnishing  the  illus- 
trations here  given. 

The  translators  have  omitted  some  20  pages  of  the  German 
text,  devoted  to  an  elaborate  argument  to  prove  that  secondary 
batteries  are  superior  to  primary  galvanic  cells  as  a  proximate 
source  of  electricity  for  electrochemical  analysis.  They  con 
sider  that  fact  as  fully  established  in  the  minds  of  all  who  are 
likely  to  make  use  of  their  work. 

It  may  be  as  well  to  say  a  few  words  in  this  connection  as 


€0  QUANTITATIVE    ANALYSIS   BY   ELECTROLYSIS. 

to  the  source  of  current.  Granted  that  the  secondary  battery 
is  the  best  proximate  source,  the  question  remains  as  to  the 
charging  of  these  batteries.  In  the  great  majority  of  cases, 
arrangements  can  be  made  for  using  a  current  that  is  main- 
tained for  other  purposes,  either  sending  out  the  cells  to  be 
jcharged,  or  charging  in  the  laboratory,  using  resistances  or 
transformer,  according  to  suggestions  elsewhere  in  this  work. 
Where  recourse  must  be  had  to  a  source  contained  within  the 
laboratory,  it  may  be  possible,  in  technical  laboratories,  to 
have  a  small  dynamo  run  by  a  belt  from  shafting  belonging 
to  the  factory. 

In  case  110  dynanlo  current  is  at  hand,  either  primary  bat- 
teries or  a  thermopile  may  be  employed.  A  chemist  in  so 
isolated  a  location  could  either  prepare  a  sufficient  number  of 
Daniell  cells  of  the  "gravity"  type,  perhaps  making  large- 
sized  cells  by  using  pails  of  moulded  fiber,  now  universally 
sold,  as  containers,  or  could  with  little  trouble  and  expense 
construct  a  thermopile  of  the  Paget  type  that  he  would  find 
sufficient  for  his  needs. — Trans.] 

PHYSICAL  METHODS  OF  PRODUCING  THE 
CURRENT. 

ELECTROMAGNETIC   MACHINES. 

For  electrochemical  processes,  uniform  continuous  cur- 
rents of  great  quantity  (low  intensity)  are  required. 

The  author  used  for  electrolytic  analysis,  during  the  years 
1881-5,  a  magneto-electric  machine  made  by  Siemens  & 
Halske  of  Berlin. 

A  pulley  was  fixed  on  the  nxis  of  this  machine,  and  con- 
nected to  a  second  pulley  on  a  counter-shaft.  The  counter- 
shaft carried  a  cone  pulley  with  5  steps  of  30,  25.  20.  15,  and 
10  cm.  diameter,  corresponding  to  a  similar  cone  pulley  on  a 
second  counter-shaft.  This  second  counter-shaft  was  provided 
with  fixed  and  loose  pulleys,  and  was  directly  connected  with 


PHYSICAL    METHODS    OF    PRODUCING  THE  CURRENT.    61 

the  source  of  power.  The  change  of  connection  of  the  cone 
pulleys,  therefore,  changed  the  velocity  of  revolution  of  the 
magneto-electric  machine. 

The  observed  velocities  of  the  machine,  with  this  arrange- 
ment, were  TOO,  500,  300,  200,  and  100  revolutions  per 
minute. 

To  control  still  more  closely  the  strength  of  the  current, 
a  regulator  was  inserted  provided  with  resistance-spirals  and  6 
contacts,  giving  resistances  of  0.01,  0.02,  0.06,  0.6,  1.45,  and 
3  ohms ;  thus  the  machine  was  made  available  for  all  deter- 
minations and  separations. 

This  arrangement  is  adapted,  as  will  be  seen,  to  carry 
on  simultaneously  only  similar  determinations ;  it  is  not 
possible,  e.g.,  to  determine  together  iron  and  antimony  or 
copper. 

The  firm  of  Siemens  &  Halske  has  constructed,  for  the 
laboratory  of  the  author,  an  apparatus  which  allows  a  large 
number  of  the  most  unlike  determinations  to  be  carried  on 
together  without  interference.  The  action  of  the  apparatus 
depends  essentially  on  an  arrangement  by  which  the  full 
current  of  the  machine  is  sent  through  an  artificial  resistance 
with  many  subdivisions,  and  the  tension  of  these  subdivisions 
is  kept  constant ;  that  is,  each  subdivision  has  a  constant 
known  tension,  which  remains  unchanged,  if  a  side  current 
of  comparatively  little  strength  is  taken  to  carry  on  a  deter- 
mination. 

Before  describing  the  details  of  the  apparatus,  it  must  be 
premised  that  the  current  is  produced  by  a  dynamo  machine 
of  the  form  shown  in  Fig.  25.  This  machine,  at  1,000  revo- 
lutions per  minute,  furnishes  a  current  of  60  amperes  with  a 
tension  of  10  volts ;  it  requires  to  run  it  something  more 
than  one-horse  power. 

Siemens  &  Halske  describe  the  action  of  the  dynamo 
as  follows:  A  current  is  produced  in  a  closed  electric  con- 


62  QUANTITATIVE   ANALYSIS   BY    ELECTROLYSIS. 

ductor  when  a  portion  of  it  is  passed  between  opposite  poles, 
which  may  be  developed  in  fixed  or  in  moving  masses  of  iron. 
The  direction  of  this  current  depends  on  the  position  of  the 
magnetic  poles  with  reference  to  the  direction  of  the  motion. 
The  conductor,  by  the  motion  of  which  the  electric  current 
is  produced,  is  insulated  copper  wire  wound  in  several  divi- 


FIG.  34. 

sions  of  many  turns  each  about  an  iron  core,  so  as  to  cover  it 
completely,  even  on  the  faces.  This  core  is  a  hollow  cylinder 
of  soft  iron  wires  or  plates,  which  revolves  on  an  axis  passing 
longitudinally  through  it  (Fig.  35,  nn,  ss').  Partly  sur- 
rounding this  hollow  cylinder  on  either  side,  and  conforming 
to  it  in  shape,  are  iron  bars,  "IW,  SS7,  the  straight  project- 


PHYSICAL   METHODS    OF   PRODUCING   THE   CURRENT.    63 


South 


North 


ing  portions  of  which  are  wound  with  insulated  copper  wire, 
and  connected  by  the  bars  m  and  O,  thus  forming  horseshoe 
electro- magnets,  NmS  and 
N'OS',  with  their  similar 
poles  opposite  to  each  other. 

By  the  action  of  the 
electro  -  magnets,  powerful 
opposite  magnetic  poles  are 
formed  in  the  iron  bars  to 
the  right  and  left  of  the 
rotating  wire-covering  of 
the  core. 

The  iron  core  becomes, 
by  induction,  a  transverse 
magnet  always  opposing  its 
poles  to  those  of  the  outer 
electro  -  magnets.  The  in- 
termediate spaces,  in  which 
revolves  the  wire  cylinder 
covering  the  core,  become 
magnetic  fields  of  great  in- 
tensity. Every  revolution 
of  the  wire  cylinder  pro- 
duces in  each  turn  of  the 
wire,  as  it  passes  through  the  two  magnetic  fields,  two  cur- 
rents in  opposite  directions.  By  means  of  a  commutator 
which  is  connected  in  a  peculiar  manner  with  the  single  coils, 
a  continuous  current  (constant  in  one  direction)  is  produced 
from  the  combined  action  of  these  single  alternating  currents. 
The  commutator  C  (Fig.  34)  consists  of  a  number  of  insulated 
copper  plates,  which,  taken  together,  form  a  cylinder  sur- 
rounding the  axis  of  the  iron  core  and  revolving  with  it. 
Brushes  of  copper  wire  transmit  the  current  from  the  com- 
mutator to  the  wire  that  forms  the  circuit. 


FIG.  35. 


64  QUANTITATIVE  ANALYSIS   BY   ELECTROLYSIS. 

According  to  the  fundamental  principle  of  the  dynamo, 
the  electric  current  which  is  produced  is  itself  utilized  for 
strengthening  the  magnetism  which  is  necessary  to  the 
machine,  the  weak  magnetism  remaining  in  the  iron  being 
sufficient  to  begin  the  action  when  the  machine  is  started. 
The  current,  to  this  end,  traverses  the  wires  which  are  wound 
about  the  electro- magnets,  as  well  as  the  external  circuit  in. 
which  it  is  utilized. 


THERMO-ELECTRIC    PILES. 

The  principle  upon  which  the  construction  of  this  form 
of  apparatus  is  based,  is  that  at  the  points  where  two  metals 
are  soldered  together  a  difference  of  potential  arises,  if  the 
junctions  are  maintained  at  different  temperatures.  Of  the 
various  forms  of  thermo-electric  piles  which  have  been  brought 
before  the  public,  those  of  diamond,  Noe  and  Giilcher  have 
found  practical  application. 

diamond's  pile  (shown  in  Figs.  36  and  37)  is  built  up 
of  a  large  number  of  elements,  each  consisting  of  a  bar  of  an 
antimony  and  zinc  alloy,  and  a  strip  of  tinned  sheet-iron ;  the 
iron  strips  are  fastened  to  the  bars  as  shown  in  Fig.  38,  thus 
serving  to  connect  the  elements.  Both  the  single  elements 
and  the  superimposed  rings  of  elements  are  separated  by 
layers  of  asbestus. 

Binding- screws  are  attached  to  the  poles  of  each  ring 
of  elements.  The  current  is  produced  by  heating  with 
illuminating  gas  which  burns  from  a  perforated  cylinder 
of  clay  or  porcelain,  standing  in  the  middle  of  the  pile  (Fig. 
39,  one-third  natural  size).  This  tube-burner  is  cemented 
in  the  cylinder  with  a  mixture  of  powdered  asbestus  and 
water-glass,  and  can  be  replaced  in  case  of  accidental  break- 
age. To  keep  the  flow  of  gas  constant,  and  prevent  exces- 


THERMO-ELECTRIC    PILES. 


65 


Fio.  37. 


66 


QUANTITATIVE  ANALYSIS   BY    ELECTROLYSIS. 


sive  heating  of  the  burner,  the  gas  is  first  passed  through  a 
regulator  filled  with  water  (r,  Fig.  36),  the  valve  of  which 
partly  closes  the  orifice  when  the  pressure  rises,  and  opens 
it  wider  when  the  pressure  falls.  The  water  in  the  regu- 


FIG.  38. 


FIG.  39. 


lator  must  be  replaced  as  it  evaporates.  The  current  attains 
its  full  strength  when  the  gas  has  been  burning  about  one 
hour. 

After  using  the  pile,  care  must  be  taken  not  to  cool  the 
tube-burner  too  quickly.  To  this  end,  the  cylinder  opening 
at  C  (Fig.  37)  is  first  closed  with  an  iron  plate  d ;  after  that 
the  cock  is  closed. 

The  elements  of  Noe's  thermopile  are  rods  of  an  alloy  con- 
taining 63  per  cent  antimony  and  37  per  cent  zinc,  about  7 


THERMO-ELECTRIC   PILES.  67 

mm.  in  diameter  and  27  mm.  long  (Fig.  40),  to  each  of  which 


FIG.  42 


is  attached  a  smaller  pointed  iron  rod  (e)  to  conduct  the  heat 


68  QUANTITATIVE  ANALYSIS   BY   ELECTROLYSIS. 

to  it.  The  elements  are  arranged  in  a  circle,  on  a  ring  of 
ebonite,  with  the  iron  point  resting  on  a  plate  which  serves 
to  spread  the  flame  of  the  gas-burner  (Fig.  41).  The  con- 
nection of  the  elements  by  German- silver  strips,  nn,  etc. ,  is 
shown  in  Fig.  42.  The  elements  are  soldered  to  copper 
plates  set  in  a  circle,  which  are  bent  in  spiral  form,  and 
serve  to  support  the  elements,  and  also  to  cool  their  outer 
ends.  Fig.  43  gives  a  general  view  of  the  Noe  thermopile. 
If  it  is  only  moderately  heated,  the  air  cools  it  sufficiently ; 
if  more  strongly  heated,  it  must  be  placed  in  a  vessel  of 
water. 


FIG.  43. 


According  to  v.  Waltenhofen,  a  pile  of  128  elements  in 
4  groups  of  32  each  is  equal  in  electromotive  force  to  about 
2  Daniel!  cells. 

Kecently  a  thermopile  of  new  construction,  devised  by 


THERMO-ELECTRIC    PILES.  69 

Oiilcher,  has  proved  especially  adapted  to  the  requirements 
of  electrolysis.  Along  the  upper  side  of  the  pipe  which  con- 
ducts the  gas,  and  which  extends  lengthwise  through  the 
apparatus,  are  situated  two  rows  of  nickel  tubes  having  at  the 
tops  small  openings  which  permit  the  escape  of  the  gas. 
On  the  upper  ends  of  the  nickel  tubes,  where  the  flames  are 
ignited,  are  soldered  plates  of  an  antimony  alloy,  from  which 
copper  strips  extend  to  the  bottom  of  the  adjoining  nickel 
tubes,  so  that  considerable  differences  of  temperature  exist 
between  the  top  of  one  tube  and  the  bottom  of  the  next. 


FIG.  44. 

Fig.  44  shows  an  apparatus  of  the  model  constructed  by  the 
firm  of  Julius  Pintsch  (Leipzig).  This  thermopile,  consuming 
170  liters  of  gas  per  hour,  furnishes  an  electromotive  force 
of  4  volts  and  has  an  internal  resistance  of  0.6-0.7  ohm,  so 
that  it  may  be  used  for  charging  accumulators. 

Wing-like  plates  of  sheet  metal  are  attached  to  each  of 
the  nickel  tubes,  and  from  these  fractional  parts  of  the  entire 
tension  of  the  apparatus  may  be  taken  off. 

The  constancy  of  the  tension  of  the  thermopile  depends 
upon  the  uniformity  of  the  gas-pressure,  which  in  cities 
is  often  liable  to  great  fluctuations.  Such  a  condition  of 


70  QUANTITATIVE   ANALYSIS   BY    ELECTROLYSIS. 

things  may  often  be  extremely  deceptive,  as  in  the  case,  for 
example,  where  an  experiment  is  continued  unobserved 
throughout  the  night.  Of  the  many  regulators  which  have 
been  devised  to  prevent  these  variations  in  the  gas-pressure, 
that  constructed  by  Danueel  (Gottingen)  excels  all  others 
in  simplicity  of  construction  and  operation,  as  well  as  in 
precision.* 

The  arrangement  of  the  apparatus  is  readily  seen  from 
the  adjacent  sketch  (Fig.  45).  The  solenoid  S 
receives  its  current  directly  from  the  thermopile, 
or,  if  desired,  from  the  pole- clamps  of  the  bath. 
It  iits  exactly  over  the  glass  tube  which  encloses 
the  apparatus,  and  draws  downward  the  magnet 
M  on  an  increase  of  the  tension.  The  opposing 
force  is  furnished  by  a  spring  so  arranged  that  it 
will  not  be  too  greatly  stretched  by  the  weight 
of  the  core,  and  still  may  respond  to  slight 
attraction.  The  spring  is  attached  at  its  upper 
end  to  a  thread  which  is  connected  with  an  ad- 
justing mechanism  plainly  shown  in  the  drawing, 
FIG.  45.  by  which  the  tension  of  the  spring  can  be  easily 
increased.  On  the  iron  core  is  hung  the  plate  of  the  plate 
valve.  The  gas  flows  in  through  the  base  B,  and  out  to  the 
thermopile  through  A.  C  is  an  adjustable  screw  ending  in  a 
cone,  which  is  used  for  closing  a  second  passage  for  the  gas 
from  B  to  A,  so  far  that  the  current  of  gas  passing  through 
is  just  sufficient  to  keep  the  lights  of  the  thermopile  from 
being  extinguished  when  the  valve  is  completely  closed.  The 
thermopile  is  provided  with  an  attachment  for  regulating  the 
air-supply,  in  order  to  prevent  the  snapping  back  of  the  flames 
when  the  gas-supply  is  low.  The  apparatus  is  mounted  on  a 

*  Ztschr.  f.  Elektrochemie,  1896-97,  p.  81. 


,7 


FIG.  46. 


FIG.  47. 


FIG.  48. 


To  face  page  71. 


THERMO-ELECTRIC    PILES.  71 

base  provided  with  adjusting-screws.  On  this  is  a  regulating 
resistance  which  allows  any  desired  tension  to  be  taken  off 
direct  from  the  apparatus.  The  thermopile,  which  in  con- 
trast to  most  other  sources  of  current  can  stand  short-cir- 
cuiting, is  short-circuited  through  a  variable  resistance  of 
coiled  wire,  and  by  the  adjustment  of  a  curved  lever  connected 
with  this  the  desired  tension  is  obtained.  Varying  the  ten- 
sion in  this  manner  is  far  more  convenient  than  the  usual 
method  of  attaching  clamps  to  the  vanes  of  the  separate 
thermo-elements. 

The  Giilcher  thermopile,  in  combination  with  accumulators, 
appears  to  be  particularly  suited  to  the  requirements  of  small 
electrolytic  experiments.  An  especially  convenient  arrange- 
ment of  this  sort  is  described  by  K.  Elbs.* 

[Another  thermopile  worthy  of  mention  is  that  invented 
by  Dr.  Leonard  Paget  of  New  York.  Although  patented, 
it  has  not  been  made  in  commercial  quantities  and  put  on  the 
market,  and  any  chemist  or  electrician  is  free  to  prepare  and 
use  one.  The  inventor  has  expressly  authorized  the  trans- 
lators to  make  this  statement,  and  will  answer  any  inquiries 
made  through  them. 

The  Paget  thermopile  is  very  simple  in  construction,  so 
that  it  can  be  made  by  or  under  the  direction  of  any  chemist 
or  electrician,   wherever  the  services  of  an  ordinary  black 
smith  or  similar  metal-worker  are  to  be  procured. 

It  consists  of  thin  annular  disks  of  copper  and  German 
silver,  buckled  or  dished  (Fig.  46),  and  placed  alternately,  one 
above  the  other,  upon  the  gas-tube  g  (Fig.  47),  so  that  adjacent 
disks  are  held  in  contact  by  their  own  elasticity.!  The  whole 
system  is  held  between  annular  iron  plates  pp  (Fig.  48)  with 

*  Chem.  Ztg.,  1893,  pp.  66  and  97. 

f  The  gas-tube  g  in  this  small  pile  is  conveniently  made  of  asbestus 
sheet,  shaped  into  tube  form  about  a  stick  of  the  desired  diameter. 


72  QlANTri  ATIVE   ANALYSIS   BY    ELECTROLYSIS. 

asbestus  washers  ww,  by  three  long  bolts  b  ;  these  bolts  are 
prolonged  to  serve  as  legs.  Heat  is  supplied  by  a  Bunsen 
burner.  The  disks  are  some  3  or  4  inches  in  diameter,  with 
an  opening  1  inch  in  diameter. 

This  thermopile  can  be  taken  apart  at  any  time  by  re- 
moving three  nuts,  and  the  contact  edges  of  the  disks  bright- 
ened by  sand-papering.  This  will  be  found  desirable  after 
several  weeks'  use,  and  can  be  done  in  an  hour  by  unskilled 
labor. 

A  larger  form,  used  with  great  success  in  copper  deter- 
minations at  the  Chicago  Copper  Refining  Works,  consisted 
of  disks  8  or  more  inches  in  diameter,  with  a  3-ii:ch  opening, 
and  was  heated  by  charcoal,  the  gas-tube  g  being  furnished 
with  a  simple  grate  and  extended  upward  to  produce  a 
draught.  This  latter  form  has  been  used  in  the  works  re- 
ferred to,  in  preference  to  an  available  side  current  from  a 
dynamo. 

Fig.  49  shows  a  still  larger  form,  heated  by  coke  on  the 
base-burning  principle,  which  was  constructed  and  used  in 
1892. 

The  tire-bnx  is  1  foot  in  diameter.  The  annular  copper 
plates  c  are  extended  to  form  cooling  plates ;  all  air  used  in 
combustion  being  drawn  over  them,  as  indicated  by  arrows. 
The  cylinder  was  2  ft.  high,  with  8  pairs  of  elements  to  the 
inch,  and  the  output  in  actual  work  was  11  volts  and  72  am- 
peres. 

Various  modifications  will  suggest  themselves  under 
special  conditions,  e.g.,  the  adaptation  of  the  size  of  the  disks 
to  the  use  of  one  of  the  more  powerful  oil  lamps  as  a  source  of 
heat. 

The  sheet  metal  used  for  the  disks  in  the  smaller  sizes  is 
Jg-  of  an  inch  in  thickness.  About  35  pairs  of  the  smaller 
size  described  give  an  E.M.F  of  1  volt,  so  that  to  charge 
two  secondary  cells  would  require  about  200  pairs. 


REGULATION  OF  THE  CURRENT. 


73 


FIG.  49 


REGULATION   OF   THE   CURRENT. 

The  relation  of  the  current  strength  to  the  resistance  is 
explained  by  Ohm's  Law ;  from  which  it  follows  that  it  is 
never  possible  to  alter  either  one  of  the  two  quantities  in- 
dependently of  the  other.  Variation  of  the  current  strength 
as  well  as  of  the  tension,  as  has  been  previously  explained,  is 
of  the  greatest  importance  for  quantitative  electrolysis,  and 
the  means  at  hand  for  the  accomplishment  of  this  are 
numerous. 

"When  accumulators  or  primary  elements  are  employed, 
the  tension  may  be  varied  by  connecting  a  greater  or  less 


74 


QUANTITATIVE   ANALYSIS   BY   ELECTROLYSIS. 


number  of  them  in  series.     If  one  element  has  a  tension  of  a 
volts,  then  n  elements  in  series  give  na  volts. 

In  the  case  of  dynamos,  the  number  of  revolutions  of  the 


FIG.  50. 

armature,  as  well  as  the  number  of  windings  of  the  magnet, 
has  a  great  influence,  so  that  by  an  alteration  of  either  of 
these  the  most  varied  tensions  can  be  obtained. 

A  convenient  method  of  altering  the  tension,  which  is 
applicable  to  every  source  of  current,  is  the  introduction  of  a 
shunt  circuit,  or  the  use  of  resistance  in  the  main  circuit. 

The  theory  of  the  shunt  circuit  is  based  upon  the  follow- 
ing considerations :  Suppose  that  the  positive  and  negative 
poles  of  a  source  of  current  are  connected  by  a  wire  100 
meters  in  length,  and  that  the  electromotive  force  is  1 0  volts. 
Then  along  the  100  meters  of  wire  a  fall  in  tension  of  10 
volts  will  take  place,  which  will  be  proportional  to  the  resist- 
ance of  the  connecting  wire.  If  the  wire  is  of  equal  cross- 
section  throughout  and  of  uniform  material,  the  resistance  and 
correspondingly  the  fall  in  tension  will  be  proportional  to  the 
length.  The  fall  in  tension  for  every  ten  meters  will  there- 


REGULATION  OF  THE  CURRENT.          75 

fore  be  1  volt,  for  every  1  meter  0. 1  volt,  etc.  If  a  branch 
circuit  be  now  attached  to  two  points  about  20  meters  apart 
this  will  have  an  electromotive  force  of  about  2  volts,  and  in 
this  manner  any  desired  tension  within  the  limit  of  the  source 
of  current  can  be  obtained.  Of  course  at  the  same  time  a 
corresponding  change  in  the  current  strength  takes  place  ac- 
cording to  Ohm's  Law,  but  since  the  current  strength,  at  a 
given  tension  in  the  branch  circuit,  depends  upon  the  resist- 
ance of  the  electrolytic  cell,  this  latter  is  variable,  and  with  it 
the  current  strength  with  the  given  tension. 

The  simple  apparatus  described  in  the  following  has  been 
constructed  by  the  author  for  this  purpose.  A  plan  of  it 
appears  in  Fig.  50. 

The  current  from  the  battery  enters  at  &,  circulates  through 
the  German-silver  resistance  N",  and  returns  to  the  battery 
through  b.  In  making  electrolytic  determinations  the  plati- 
num dishes  serving  as  negative  electrodes  may  be  attached  to 
any  one  of  the  binding- screws  1-20,  while  the  platinum  foils 
serving  as  positive  electrodes  are  attached  to  the  binding-screws 
marked  with  the  4-  sign.  The  apparatus,  therefore,  is  suited 
to  carry  on  eight  different  determinations  simultaneously. 
Its  value  for  analytical  purposes  is  shown  by  the  following 
experiments.  To  determine  directly  the  current  strength  at 
the  binding- screws  1-20,  150  cc  of  a  15$  copper  sulphate 
solution  was  placed  in  each  of  6  platinum  dishes  of  equal 
size,  copper  anodes*  were  used,  and  the  current  was  passed 
for  7.  minutes  in  each  case. 

The  current  was  produced  by  a  battery  of  5  Bunsen  cells. 

As  already  stated,  the  current  strength  in  the  shunt  cir- 
cuit is  proportional  to  the  tension,  if  the  resistance  remains 


*  6  cm.  in  diameter,  2  mm.  thick.     The  diameter  of  the  platinum  dishes 
was  9  cm.,  the  distance  of  the  electrodes  from  each  other  2.5  cm. 


76  QUANTITATIVE   ANALYSIS   BY    ELECTROLYSIS. 

constant.  This  fact  permits  of  accurate  determination  of  the 
changes  in  the  tension  from  the  measured  variations  of  the 
current  strength. 


FIEST  EXPERIMENT. 

g.  Cu.     Amperes. 

Binding-screw  1 0.5064  =  3.75 

«  2  .    *    ...  0.3507  =  2.617 

«  3 0.2873  =  2.085 

«  4  .....  -0.2358  =  1.711 

«  5 0.1857  =  1.348 

«  6 0.1453  =  1.054 

u  7 0.1341  =  0.973 

«  8  0.1128  =  0.818 


SECOND  EXPERIMENT. 

g.  Cu.     Amperes. 

Binding-screw    7  .....  0.2213  =  1.606 

«  8 0.1622  =  1.177 

«  9 0.1356  =  0.984 

«  10 0.1083  =  0.786 

«  11 0.0846  =  0.614 

«  12 0.0744  =  0.576 

«  13 0.0506  =  0.367 

«           14  ,  0.0410  =  0.225 


REGULATION  OF  THE  CURRENT.          77 

THIRD  EXPERIMENT. 

g.  Cu.     Amperes. 

Binding-screw  13 0.1983  =  1.446 

14  -.    ....  0.1304  =  0.946 

"           15  .  j.    ,    .    .  0.1276  =  0.926 

"           16  .    .    .    .     .  0.0855  =  0.620 

"           17  .....  0.0605  =  0.439 

"  18 0.0385  =  0.280 

"           19  .....  0.0314  =  0.227 

"           20  ...     .     .  0.0136  =  0.098 

From  a  number  of  quantitative  determinations,  which 
were  made  by  Norrenburg  with  this  apparatus,  the  following 
are  selected :  — 

SERIES  I. 

The  apparatus  was  attached  to  a  battery  of  5  Bunsen  cells, 
and  eight  iron  determinations  were  made  simultaneously. 
The  precipitation  was  complete  in  6  hours. 

Taken  Found  Binding          cc. 

FeSO4,2(NH4)2SO4?6H2O.  Fe.  Screw.      OH  Gas. 


1.2918 

g- 

0.1846  g. 

=  14.30^  \ 

(  24.0 

1.4360 

u 

0.2059    " 

=  14.33     L 

2     1  25.0 

1.1926 

(1 

0.1708    " 

=  14.32    j 

(24.0 

1.1964 

it 

0.1700    " 

=  14.30    } 

(  16.8 

1.2945 

u 

0.1851    " 

=  14.30     V 

3     •]  16.6 

1.3218 

tt 

0.1892    " 

=  14.31    ) 

(  17.2 

1.2931 
1.3255 

u 

(4 

0.1854    " 
0.1895    " 

=  14.34     ) 
=  14.30     ) 

j  13.2 
4     J13.4 

*  The  measurements  given  here,  taken  from  earlier  experiments  con- 
ducted with  tlie  oxy hydrogen  gus  voltameter,  now  entirely  abandoned,  per- 
mit of  the  recognition  of  the  correct  variations  of  the  intensity,  since  the 
same  instrument  was  used  in  all,  and  in  each  case  but  a  short  time. 


78  QUANTITATIVE   ANALYSIS   BY    ELECTROLYSIS. 

At  the  close  of  the  experiment  the  battery  (without  resist- 
ance) still  yielded  50  cc  OH  gas. 


SERIES  II. 

Three  nickel  and  five  copper  determinations  were  con- 
ducted simultaneously.  The  current  from  5  Bunsen  cells 
yielded  65  cc  OH  gas. 


Taken 
Nickel  Ammon.  Sulph. 

1.2848  g. 
1.4341  " 
1.2008  « 

Copper  Sulphate. 
1.1531   g. 

0.9787  " 
1.0092  " 

0.9938  " 
1.0088  " 


Found 
Nickel. 

0.1963  g.     =  15. 
0.2201    "     =15.35 
0.1842    "     =  15.34 

Copper. 

0.2910  g.  =  25.23; 
0.2476  u  =  25.30 
0.2556  "  =25.32 


Binding       cc. 
Screw.   OH  Gas. 


0.2515 

0.2550 


=  25.30 
=  25.27 


SERIES  III. 

This  established  the  applicability  of  the  process  to  the 
simultaneous  determination  of  nickel,  antimony,  and  copper. 
The  number  of  analyses  again  was  eight.  The  battery,  of 
5  Bunsen  cells,  yielded  65  cc  OH  gas  per  minute. 


REGULATION    OF   THE   CURRENT 


79 


Found 
Nickel. 

Binding 
Screw. 

cc. 
OH  Gas. 

15.30JH 

(  21.0 

15.27 

3 

•]  22.0 

15.32     j 

(  22.0 

Antimony. 

71.44$  \ 
71.47    l 

9 

j  i:°o 

71.49    ) 

\    1.0 

Copper. 
25.30     I 
25.30     j 

7 

)    3.6 
j     3.6 

Taken 
Nickel  Ammonium  Sulphate. 

1.3022  g. 
1.1520  " 
1.4391  " 

Antimony  Trisulphide. 
0.1609  gm. 
0.1691     " 
0.1626     " 
Copper  Sulphate. 
0.2527    g. 
0.2550     " 


The  current  strength  of  the  battery  at  the  close  of  the  last 
two  series  was  about  half  that  at  the  beginning. 

These  experiments  show  plainly  the  practical  advantage 
of  this  rheostat.  To  perform  eight  iron  determinations 
(Series  I )  simultaneously  without  this  rheostat  would  require 
8  ordinary  rheostats  and  at  least  16  cells.  For  three  nickel 
and  five  copper  determinations  would  be  needed  16  Bunsen 
cells  and  8  rheostats,  or  6  cells,  3  rheostats,  and  5  Meidinger 
batteries  of  3  or  4  cells  each,  the  latter  for  the  copper  deter- 
minations. The  conditions  would  be  similar  with  the  third 
series. 

[Prof.  Edgar  F.  Smith  uses  a  simple  apparatus  (Fig.  51), 
the  accompanying  figure  and  description  of  which  he  kindly 
permits  the  translator  to  copy : 

"The  writer  has  for  some  time  employed  a  much  simpler 
current- reducer,  which  has  the  advantage  of  cheapness  and 
ready  construction  to  recommend  it.  It  consists  of  a  light 
wooden  parallelogram,  about  6  feet  in  length.  Extending 
from  end  to  end,  on  both  sides,  is  a  light  iron  wire,  measuring 


80 


QUANTITATIVE  ANALYSIS   BY   ELECTROLYSIS. 


in  all  about  500  feet.  With  the  binding-posts  at  a  and  &,  and 
a  simple  clamp,  it  is  possible  to  throw  in  almost  any  resistance 
that  may  be  required.  It  answers  all  practical  purposes." — 
' c  Electro-chemical  Analysis. ' ' — Trans.  ] 


FIG.  51. 

A  direct  alteration  of  the  current  strength  is  in  general 
brought  about  by  the  introduction  of  resistance  into  the  main 
circuit.  It  may,  however,  take  place  in  the  elements  them- 
selves, whereby  several  elements  are  connected  in  parallel,  i.e., 


REGULATION  OF  THE  CURRENT.          81 

the  similar  poles  connected  with  one  another.  This  has  the 
effect  of  increasing  the  electrode  surface,  and  consequently 
decreases  the  internal  resistance. 

With  dynamos  such  a  parallel  coupling  is  of  course  not 
practical. 

A  much  more  general  method  for  varying  the  current 
strength  is  through  alterations  of  the  external  resistance,  which 
can  be  carried  out  to  a  certain  extent  in  the  cell  itself,  by 
placing  the  electrodes  further  apart  or  by  the  addition  to  the 
solution  of  substances  which  increase  or  decrease  its  conduc- 
tivity. Outside  of  the  cell  "  rheostats  "  are  employed.  These 
are  metallic  resistances,  portions  of  which  may  be  switched 
in  or  out  as  desired. 

Of  the  innumerable  different  models,  many  of  which  are 
described  in  the  text-books  of  physics,  only  one,  of  a  con- 
struction similar  to  those  used  for  a  long  time  in  the  Aachen 
laboratory,  will  be  mentioned. 

For  the  reduction  of  the  current  strength,  plug  rheostats 
deserve  special  recommendation.  As  ordinarily  constructed, 
these  are  ill  adapted  to  laboratory  use,  for  the  plugs  are 
quickly  attacked  by  acid  vapors  from  the  cells  or  the 
vapors  of  the  laboratory,  and  the  resistance  introduced  is 
thus  changed.  The  ordinary  apparatus  has  also  the  fault 
that  the  plugs  are  liable  to  become  loose.  Both  difficulties 
are  met  by  the  use  of  mercury  contacts,  instead  of  plugs,  to 
connect  the  metallic  plates.  Fig.  52  shows  the  arrangement 
of  such  a  rheostat.*  By  inserting  the  contact  bars  CC  in 
the  mercury  cups  or  removing  them,  any  resistance  from  0.5 
to  80  ohms  may  be  inserted  by  intervals  of  0.5  ohms. 

The  following  results  of  experiment  show  the  action  of 
this  rheostat.  A  current  from  three  Bunsen  cells  yielding 

*  This  rheostat  is  made,  at  the  author's  suggestion,  by  Frans  Brothers  in 
WiiMsiedel. 


82  QUANTITATIVE  ANALYSIS   BY    ELECTROLYSIS. 

in  the  voltameter  28  cc  oxyhydrogen  gas  per   minute  was 
reduced  as  follows: — 


Ohms  inserted. 

cc  Oxyhydrogen 
Gas  per  Minute. 

Ohms  inserted. 

cc  Oxyhydrogen 
Gas  per  Minute. 

0.5 

16.00 

15.0 

2.20 

1.0 

12.50 

20.0 

1.30 

1.5 

9.75 

30.0 

1.10 

2.0 

7.00 

40.0 

0.80 

3.0 

6.00 

50.0 

0.70 

4.0 

5.00 

60.0 

0.60 

5.0 

4.90 

70.0 

0.50 

7.5 

4.00 

80.0 

0.45 

10.0 

3.50 

A  current  of  16  cc  Oxyhydrogen  gas  per  minute  (yielded 
by  two  Bun  sen  cells)  was  reduced  by  40  ohms  to  0.4,  and  by 
80  ohms  to  0.15  cc  oxyhydrogen  gas. 

More  recently  the  author  has  used  the  simplified  form  of 
rheostat  shown  in  Fig.  53,  in  which  brass  plates  are  dispensed 
with,  and  the  contact  with  the  German-silver  coils  is  made 
directly  by  mercury. 

The  following  results  of  experiment  show  how  constant 
is  the  current  from  Bunsen  cells  when  a  rheostat  is  used.  In 
the  separation  of  antimony  from  tin,  the  current  from  two 
Bunsen  cells  was  reduced  to  0.6  and  2  cc  oxyhydrogen  gas 
per  minute. 

Columns  A  and  B  give  the  strength  of  the  two  Bunsen 
elements ;  columns  C  and  D,  that  obtained  by  use  of  the 
rheostat. 

A  and  C  were  measured  before  the  experiments;  B  and 
D,  after  them  (lapse  of  time,  14  hours). 


FIG.  52. 


FIG.  53. 


To  face  pa  (je  8~l. 


THE   PROCESS    OF  ANALYSIS. 


83 


A. 

B. 

C. 

D. 

cc  OH  Gas. 

cc  OH  Gas. 

cc  OH  Gas 

cc  OH  Gas. 

17 

16.0 

0.6 

0.3 

24 

19.0 

0.6 

0.4 

18 

11.5 

0.6 

0.3 

17 

15.5 

0.6 

0.4 

THE   PROCESS   OF   ANALYSIS. 

The  performance  of  a  quantitative  anatysis  by  electrolysis 
requires,  above  all  things,  extreme  cleanliness.  As  it  is  impos- 
sible, in  electro-plating,  to  obtain  a  metallic  coating  on  any 
surface  which  is  not  most  carefully  cleaned  before  it  is  placed 
in  the  bath,  so  a  quantitative  analysis  cannot  be  successfully 
carried  out  unless  the  metallic  surface  serving  as  cathode  is 
previously  perfectly  cleaned  and  freed  from  fat.  The  same 
care  must  be  used  with  the  battery  connections,  the  stand 
which  serves  to  conduct  the  current,  etc. ;  otherwise  it  is 
impossible  to  avoid  the  weakening  or  breaking  of  the  current. 

It  is  plainly  desirable  that  the  surface  of  the  cathode 
should  be  large  in  order  that  the  separated  metal  may  be 
more  firmly  attached  to  it.  If  a  metal  separates  from  a  solu- 
tion in  dense  form,  as  is  the  case  in  the  electrolysis  of  double 
oxalates,  the  possibility  of  the  oxidation  of  the  metal  is 
scarcely  increased  by  enlarging  the  cathode. 

In  the  separation  of  peroxides  (e.g.,  lead  and  manganese 
peroxides),  which  are  much  less  firmly  attached,  the  size  of 
the  electrode  on  which  they  are  deposited  is  of  especial 
importance. 

It  is  not  desirable,  therefore,  to  employ  a  platinum  cruci- 
ble for  electrolytic  precipitation  if  more  than  a  few  milli- 
grams are  to  be  determined  ;  not  only  is  the  surface  of  the 


84  QUANTITATIVE   ANALYSIS   BY    ELECTROLYSIS. 

cathode  too  small,  but  the  electrodes  are  not  widely  enough 
separated  to  facilitate  the  separation  of  the  metal  in  a  dense 
form. 

For  these  reasons,  the  author  uses  as  the  negative  electrode 
a  thin  platinum  dish  of  35-37  grams  weight,  9  cm.  diameter, 
4.2  cm.  depth,  and  about  250  cc  capacity.  The  dish  has  tho 
form  shown,  in  about  one-half  natural  size,  in  Fig.  54  Dishes 
which  have  become,  in  the  course  of  time,  rough,  scratched, 
or  bent,  cannot  be  used  for  electrolysis. 

Some  metals  separate  less  readily  in  hammered  dishes 
than  in  those  which  are  spun  and  polished  on  the  lathe* 


FIG.  54. 

If,  for  instance,  hammered  dishes  are  used  for  the  reduction 
of  zinc  from  the  double  oxalate,  there  always  remains,  after 
the  solution  of  the  metal  in  acid,  a  gray,  closely  adherent 
coating  of  platinum  black,  which  is  with  difficulty  removed 
even  by  melted  potassium  hydrogen  sulphate,  and  which 
makes  further  determination  of  metals  in  the  dish  difficult. 

For  many  purposes,  as  for  example  the  determination  of 
lead  in  the  form  of  peroxide,  the  firm  adherence  of  the  pre- 
cipitate to  the  dish  can  only  be  secured  by  the  use  of  a  plati- 
num dish,  the  inner  surface  of  which  has  been  roughened  with 
a  sand-blast. 

It  is  to  be  recommended  that  under  all  circumstances  the 
dishes  used  for  electrolysis  be  reserved  exclusively  for  their 
intended  purposes. 

The  great  flexibility  of   pure  platinum   dishes   has  been 


THE   PROCESS   OF   ANALYSIS.  85 

recently  overcome  by  the  use  of  platinum -iridium  dishes. 
The  iridium,  of  which  about  10  per  cent  is  added  to  the 
platinum,  gives  a  much  greater  hardness  and  resistibility  to 
the  utensils  made  from  this  alloy  than  is  possessed  by  pure 
platinum  articles. 

As  anode  (positive  electrode),  the  author  uses  a  plate  of 
moderately  thick  platinum  foil,  about  4.5  cm.  in  diameter, 


FIG.  55,  FIG.  56. 

which  is  fastened  to  a  tolerably  stout  platinum  wire  (Fig.  55). 
It  is  desirable,  in  order  to  insure  uniformity  of  the  solution 
during  electrolysis,  to  make  a  few  holes  in  the  platinum  foil 
with  a  cork-borer.  If  this  is  neglected,  a  large  bubble  of  gas 
may  form  under  the  anode  by  the  union  of  several  smaller 
ones,  and  this  bubble,  on  escaping,  may  cause  spirting  and  loss. 
The  author  has  used  as  positive  electrode,  in  addition  to 


86  QUANTITATIVE  ANALYSIS   BY   ELECTROLYSIS. 

the  one  shown  in  Fig.  55,  a  platinum  dish  of  the  form  shown 
in  Fig.  54,  50  mm.  in  diameter  and  20  mm.  deep.  To 
secure  better  circulation  of  the  solution,  and  more  rapid  re- 
duction, the  electrode  has  five  openings  in  it.  This  form  of 
electrode  is  specially  adapted  to  the  determination  of  such 
metals  as  have  the  tendency  to  separate  in  a  spongy  state,  e.g., 
cadmium  and  bismuth. 

Two  standards  were  formerly  used,  as  shown  in  Fig.  59,  to 
support  the  two  electrodes.  The  author  substituted  a  single 
standard  (Fig.  56)  provided  with  a  metallic  ring  to  which 
three  short  contact  wires  of  platinum  are  riveted  for  the  plati- 
num dish  to  stand  on,  and  an  insulated  arm  a  of  glass  to  sup- 
port the  positive  electrode.  The  use  of  this  stand  has  the 
drawback  that  the  brass  rod  to  which  the  metallic  ring  and 
glass  arm  are  clamped  is  readily  corroded  by  the  laboratory 
vapors,  and  this  may  lead  to  the  breaking  of  contact.  The 
stand  shown  in  Fig.  57  has  given  good  service  for  a  long  time. 
King  and  arm  are  clamped  to  a  glass  rod  Gr,  and  n  is  connected 
with  the  negative  and  p  with  the  positive  pole.  The  posi- 
tive electrode  is  clamped  in  place  at  e.  If  a  platinum  cone 
is  used  instead  of  a  platinum  dish  for  the  deposition  of  the 
metal  (as  described  later),  two  arms  are  clamped  to  the  glass 
standard,  as  shown  in  Fig.  58.  This  arrangement  is  also 
convenient  when  a  metal  is  to  be  precipitated  from  an  acid 
solution ;  the  standard  with  the  electrodes  is  removed  quickly 
from  the  solution  and  plunged  into  a  vessel  of  water,  and  the 
water  is  finally  removed  from  the  negative  electrode  by  'wash- 
ing with  alcohol. 

[It  is,  of  course,  not  necessary  to  use  a  special  standard 
constituting  a  part  of  the  conductor,  as  shown  in  the  figures. 
The  platinum  dish  may  be  placed  on  the  table  or  the  base  of 
a  wooden  standard,  on  a  coil  of  platinum  or  bright  copper 
wire  which  is  connected  with  the  negative  pole  of  the  bat- 


THE   PROCESS   OF  ANALYSTS- 


87 


terj;   and   the  positive  electrode  may  be  held  in  a  wooden 
clamp  on  such  a  standard,  and  connected   directly  with   the 
wire  from  the  positive  pole.     See  also  pp.  89-92. — Trans. ,] 
When  a  platinum  dish  is  used  it  may  be  placed  on  a  metal- 


FIG.  57.  J^io.  58. 

lie  tripod  in  a  beaker,  and  the  acid  displaced  by  a  stream  of 
water  from  a  wash-bottle  after  the  reduction  is  complete. 


Fia.  59. 


88  QUANTITATIVE   ANALYSIS   BY   ELECTROLYSIS. 


FlG.  61. 


FIG.  62. 


FIG.  63. 


THE   PROCESS   OF   ANALYSIS. 


The  electrodes  used  almost  exclusively  for  copper  deter- 
minations at  the  Mansfeld  smelting- works  are  shown  in  Figs. 
59  to  64.  According  to  the  quantity  of  metal  to  be  deter- 


FIG.  64. 

mined,  the  cylinder  of  platinum  foil  shown  in  Fig.  60  (one- 
half  natural  size),  or  the  platinum  cone  shown  in  Fig.  61 
(one-fourth  natural  size)  is  used.  The  positive  electrode  is 
either  a  thick  platinum  wire  wound  in  spiral  form  (Fig.  62), 
or  has  the  form  shown  in  Fig.  63.  The  arrangement  of  the 
several  parts  is  shown  in  Figs.  59  and  64. 

[An  apparatus  described  by  v.  Malapert*  is  specially 
adapted  to  the  use  of  the  above-described  electrodes,  and 
particularly  to  carrying  on  simultaneously  several  similar 
determinations. 

As  shown  in  Fig.  65,  a  single  wooden  standard  A  sup- 
ports the  apparatus  for  several  electrolytic  determinations,  the 
lower  board  B  carrying  the  vessels  containing  the  solutions 
to  be  electrolyzed,  and  the  upper  board  C  the  apparatus  for 
directing  the  current  as  desired.  In  the  apparatus  described, 

*  Zts.  anal.  Ch.,  26,  56. 


90 


QUANTITATIVE   ANALYSIS   BY   ELECTROLYSIS. 


the  two  boards  are  18  cm.  apart,  and  the  upper  board  7  cm, 

wide. 

Fig.    66  shows,   on  a   larger    scale,  the    arrangement   for 

directing  the  current,  connected  with  each  pair  of  electrodes. 

The  two  strips  lib  of  brass  are 
1  cm.  wide,  2  mm.  thick,  and 
their  centres  3  cm.  apart.  The 
binding-screws  aa  serve  for 
the  attachment  of  the  elec- 


FIG.  65. 


FIG.  66. 


trodes ;  to  cc  are  attached  the  conducting-wires.  The  switch 
d  establishes  or  breaks  connection  between  the  two  strips 
according  as  it  is  in  the  position  shown  in  the  cut  (closed),  or 
is  moved  to  bear  on  the  curved  strip  of  hard  rubber  e  (open). 

When  the  apparatus  is  arranged,  as  shown  in  Fig.  65,  with 
the  conducting-wires  from  the  battery  connected  with  the 
end  binding-screws,  and  adjacent  binding-screws  throughout 
connected  by  wires,  the  current  passes  unhindered  so  long  as 
the  switches  are  closed.  To  insert  any  desired  number  of 
similar  solutions  for  electrolysis,  it  is  only  necessary  to  place 
the  solutions  and  electrodes  in  position,  and  open  the  corre- 
sponding switches ;  the  current  is  then  forced  to  pass  through 
the  solutions. 

If  dissimilar  determinations  are  to  be  made,  the  connecting- 
wires  between  adjacent  pairs  of  brass  strips  are  removed,  and 


THE    PROCESS    OF    ANALYSIS.  91 

the  conducting-wires  from  each  battery  in  use  are  brought 
directly  to  the  binding-screws  cc  of  one  pair  of  strips. 

To  remove  acid  solutions  without  interrupting  the  cur- 
rent, v.  Malapert  uses  beakers  of  heavy  glass  8  cm.  in  diam- 
eter and  12  cm.  high,  with  a  side  tubulure  near  the  top,  as 
shown  in  Fig.  65.  A  cork  is  inserted  in  the  hole  between 
the  brass  strips  shown  in  Fig.  66,  through  which  passes  with 
little  friction  a  glass  tube  connected  by  rubber  tubing  with  a 
reservoir  of  water.  When  the  precipitation  is  complete,  a 
stream  of  water  is  turned  on,  and  the  acid  solution  displaced, 
passing  off  through  the  tubulure.  A  common  beaker  with 
siphon  can,  of  course,  be  used. 

A  resistance  coil  of  German-silver  wire  is  shown  in  Fig. 
65  connected  to  the  pair  of  binding-screws  at  the  extreme 
right.  Any  desired  resistance  can  be  thus  conveniently 
inserted. 

An  apparatus,  made  according  to  this  description,  was 
prepared  for  use  in  the  chemical  laboratory  of  the  Pennsyl- 
vania State  College,  with  an  addition,  devised  by  the  trans- 
lator, which  makes  it  equally  convenient  when  a  platinum 
dish  is  used  as  the  negative  electrode. 

Fig.  67  shows  the  nature  of  the  addition  referred  to.  The 
brass  strip  connected  with  the  negative  electrode  is  extended 
downward,  at  the  rear  (G,  Fig.  67),  to  the  lower  board.  Here 
it  is  connected  with  the  brass  plate  H,  which  is  set  into  the 
board  B  so  as  to  be  flush  with  its  upper  surface,  and  has  a 
shallow  saucer-shaped  depression,  the  centre  of  which  is 
directly  beneath  the  binding-screw  to  which  is  attached  the 
positive  electrode.  The  plate  H  and  the  entire  strip  were 
cut,  in  the  apparatus  originally  made,  from  a  single  sheet  of 
brass. 

A  platinum  dish  placed  in  the  saucer-shaped  depression  is 
firmly  supported,  and  is  in  good  metallic  connection  with  the 


92  QUANTITATIVE   ANALYSIS    BY    ELECTROLYSIS. 

negative  pole  of  the  battery ;  the  positive  electrode  is  attached 
as  in  the  original  form  of  the  apparatus.  All  the  adjustments 
of  the  original  apparatus  are  retained,  and  the  brass  plate 
offers  no  impediment  to  the  use  of  a  beaker  with  cone-shaped 
negative  electrode,  as  shown  in  Fig.  65. —  Trans.'] 


FIG.  67. 

Herpin  uses,  for  electrolysis,  the  apparatus  shown  in 
Fig.  68.  The  platinum  dish  P  standing  on  the  tripod  F  is 
connected  with  the  negative  pole,  the  platinum  spiral  S 
(shown  separately  in  Fig.  69)  with  the  positive.  The  dish  is 
covered  with  a  glass  funnel  T  to  avoid  loss  by  spirting  of 
the  solution. 

Eiche  uses,  as  cathode,  a  platinum  cone  (Fig.  TO)  open 
at  both  ends,  having  the  form  of  a  crucible,  and  provided 


THE   PROCESS    OF   ANALYSIS. 


93 


with  a  bail.  Oblong  openings  are  made  in  the  cone  to  facili- 
tate a  uniform  concentration  of  the  liquid  during  reduction. 
The  cone  is  placed  in  a  platinum  crucible  so  that  it  is  2  to 
4  mm.  from  it.  The  whole  arrangement  is  seen  in  Fig.  71. 


FIG.  68. 


FIG.  69. 


«  To  return  to  the  consideration  of  the  actual  electrolysis: 
sulphates  are  best  adapted  to  conversion  into  double  ox- 
alates  (see  p.  5),  chlorides  less  so,  and  nitrates  entirely 
unadapted.  If  chlorides  have  been  used,  and  the  smell  of 
chlorine  is  observed  during  the  electrolysis,  ammonium  oxa- 
late  must  be  gradually  added  to  the  solution  till  the  odor 


94  QUANTITATIVE   ANALYSIS    BY    ELECTROLYSIS. 

disappears.     Sometimes  potassium  oxalate,  sometimes  ammo- 


FIG.  70. 


FIG.  71. 


mum  oxalate,  and  often  a  mixture  of  the  two,  is  used  for  the 
preparation  of  the  double  salt. 

As  hot  solutions  conduct  the  current  more  readily,  solu- 
tions are  often  heated  before  they  are  submitted  to  electro- 
lysis. In  some  cases,  however,  as  in  the  determination  of 
antimony,  it  is  necessary  that  the  solution  should  be  of  the 
ordinary  temperature. 

In  certain  determinations  and  separations,  it  is  best  to  keep 
the  solution  to  be  electrolyzed  at  moderate  heat,  not  above  50° 


THE   PROCESS    OF  ANALYSIS. 


C.  The  following  experiments  show  the  influence  of  heat  on 
the  time  required  for  electrolysis.  Nearly  equal  weights  of 
iron  and  nickel  were  precipitated,  under  as  equal  conditions 
as  possible  (strength  of  current,  concentration,  etc.),  from 
solutions  kept  respectively  at  50°  and  15°:  — 

IKON. 


Taken. 

Found. 

Strength  of  Current. 

Time. 

cc  OH  Gas. 

h.  m. 

j    (  0.2385  g  Fe2O3 

(Cold)   . 

0.2384 

11 

4  20 

'  1  0.2345  g  Fe2O3 

(Warm), 

0.2342 

11 

2  10 

n    (0.2246 

(Cold)   . 

0.2244 

10 

4  10 

'  (0.2369 

(Warm), 

0.2369 

10 

2  15 

NICKEL. 


Taken. 

Found. 

Strength  of  Current. 

Time. 

cc  OH  Gas. 

h.  m. 

I    (  0.2660  g  Ni 

(Cold)    . 

0.2660 

13 

7  25 

'  (  0.2660  g  Ni 

(Warm), 

0.2659 

13 

2  20 

n    (  0.2660  g  Ni 

(Cold)   . 

0.2661 

13 

7  30 

'  1  0.2660  g  Ni 

(Warm), 

0.2660 

13 

2  20 

It  also  results  from  the  foregoing  experiments,  that  the 
current-strength  can  be  greatly  reduced  by  the  use  of  hot  solu- 
tions, in  case  there  is  no  occasion  for  hastening  the  electrolysis. 

The  statements  in  this  book  apply  to  solutions  at  the 
ordinary  temperature,  except  when  the  contrary  is  stated. 

For  heating  the  solution  to  about  50°  (it  must  on  no 
account  be  heated  to  boiling,  else  the  reduced  metal  will 
flake  off  from  the  platinum,  and  cannot  be  determined),  the 
burner  shown  in  Fig.  72  is  used.  The  tube  of  a  Bunsen 
burner  may  also  be  unscrewed,  and  the  luminous  jet  issuing 
from  the  opening  at  the  bottom,  reduced  to  a  few  millimetres 
in  height,  used  to  heat  the  solution.  The  distance  of  the 


96  QUANTITATIVE  ANALYSIS   BY   ELECTROLYSIS. 

dish  from  the  burner  must  be  about  15  cm.  To  insure 
uniform  distribution  of  the  reduced  metal  on  the  dish,  it 
must  be  uniformly  heated.  This  is  most  simply  accomplished 
by  placing  under  the  dish  a  piece  of  thin  asbestos  paper  cut 
through  at  the  points  of  contact  of  the  dish  with  the  contact 
wires  of  the  standard.  The  use  of  asbestos  paper  also  dimin- 
ishes the  danger  of  boiling. 


FIG.  72. 


In  order  to  obtain  a  uniform,  easily  regulated  heating, 
Engels*  recommends,  as  a  result  of  investigations  conducted  in 
the  Aachen  laboratory,  the  use  of  an  asbestus  board  at  a  dis- 
tance of  2  cm.  above  which  the  dish  is  held  by  the  standard, 
and  under  which  there  is  an  ordinary  Bunsen  burner.  The 
dish  is  thus  maintained  in  an  air-bath,  the  temperature  of  which 
can  be  easily  kept  constant,  and  the  heating  is  quite  uniform. 
Such  is  not  the  case,  however,  in  the  other  methods  of  warm- 
ing. There,  at  the  points  where  the  dish  and  stand  are  in 
direct  contact,  a  higher  temperature  arises,  and  as  a  result  a 
greater  precipitation  takes  place  at  these  points  than  on  the 
other  portions  of  the  dish. 

*  Ztschr.  f.  Electrochemie,  1895-96,  p.  413. 


THE   PROCESS    OF    ANALYSIS.  97 

When  the  current  acts  for  a  long  time  it  is  impossible  to 
prevent  some  evaporation  of  the  solution,  whereby  a  part  of 
the  reduced  metal  is  exposed  to  the  action  of  water-vapor 
and  air.  To  prevent  the  oxidation  of  metal  laid  bare  by 
evaporation,  a  little  water  is  poured  from  time  to  time  on 
the  glass  cover  of  the  dish,  so  that  the  metal  remains  always 
covered  by  the  solution. 

After  precipitation  is  complete,  the  solution  remaining  in 
the  dish  is  poured  into  a  beaker,  with  care  to  avoid  loss,  the 
dish  washed  three  times  with  about  5  cc  of  cold  water,  and 
then  three  times  with  pure  absolute  alcohol.  The  dish  is 
dried  some  five  minutes  in  an  air-bath  at  70°-90°  C,  allowed 
to  cool  thoroughly  in  a  desiccator,  and  weighed. 

The  apparatus  hitherto  described  have,  without  exception, 
been  the  result  of  work  conducted  in  the  Aachen  laboratory. 
Other  forms  of  apparatus,  which  to  a  limited  extent  have 
also  found  application,  have  mostly  originated  from  v. 
Klobukow.  He  describes  a  universal  stand  (Fig.  73)  which 
carries  upon  a  rod  screwed  into  the  work- bench  all  the  ap- 
paratus necessary  for  electrolysis. 

"  If  a  platinum  dish  is  used  as  electrode  in  performing 
the  electrolysis,  then  the  ring  7?,  to  which  three  platinum 
points  are  soldered,  serves  as  a  holder  for  the  dish  s.  The 
second  electrode  E  is  clamped  into  the  holder  d,  the  latter 
being  adjustably  attached  by  means  of  the  sleeve  D  to  the 
cross-arm  TT,  of  hard  rubber.  The  path  of  the  current  is 
as  follows :  From  m  to  the  dish  s  by  means  of  the  metallic 
rod,  from  there  to  the  electrode  ^and  thence  along  d  to  the 
binding-screw  n  fastened  at  the  end. 

' '  The  conducting  wires  are  connected  to  the  rocker  TF, 
firmly  fastened  to  the  work-bench  and  coupled  into  the  cir- 
cuit from  the  source  of  current.  In  addition  there  are  wires 
attached  to  the  voltmeter,  the  ends  of  which  may  be  properly 


98 


QUANTITATIVE   ANALYSIS   BY   ELECTROLYSIS. 


connected  with  s  and  E  for  the  measurement  of  the  potential 
differences  at  the  electrodes. 

"Additional  parts  of  the  '  universal  'stand'  are:  the 
micro-burner  B,  which  serves  to  warm  the  liquid  in  s  during 
the  electrolysis,  and  the  bottle  F^  provided  with  a  siphon  of 
special  construction,  and  containing  the  liquid  to  be  used  for 


FIG. 


washing  the  metallic  precipitate  in  the  dish.  The  construction 
of  the  siphon  mentioned  is  such,  that  by  opening  the  cocks 
A,  A,,  A2  in  proper  order  the  washing  liquid  is  on  the  one 
hand  introduced  into  the  dish,  and  on  the  other  hand  the  con- 
tents of  the  dish  are  transferred  to  the  beaker  6r." 


THE   PKOCESS    OF   ANALYSIS. 


99 


The  form  of   the    electrode  E  given   by  v.  Klobukow 
differs  from  that  described  by  the  author,  in  that 
it  is  convex   on  its   lower  surface,  the  curvature 
corresponding  exactly  to  that  of  the  bottom  of  the 
dish.  FIG.  74. 

This  construction  is,  indeed,  in  accordance  with  jthe  theo- 
retical principles,  but  from  the  author's  experience  it  is  not 
necessary.  It  is  curious  that  the  metallic  precipitates  gener- 
ally have  a  better  appearance  when  flat,  rather  than  curved, 
electrodes  are  employed.  (See  Fig.  Y4.) 

For  his  universal  apparatus,  v.  Klobukow  proposes  at 
the  same  time  the  use  of  a  stirring  attachment.  With  this 


FIG.  75. 

a  slow  rotary  motion  is  given  to  the  electrode  E  by  means 
of  a  suitable  motor  connected  with  it.     (See  Fig.  75.) 


100         QUANTITATIVE   ANALYSIS   BY   ELECTROLYSIS. 


FIG.  76. 


For  the  special  purposes  of  electrolysis,  in  addition  to  the 
electrodes  and  dishes  described,  there  have  been  made  a  large 
number  of  suggestions  which  are  all  based  more  or  less  upon 
the  same  principle.  The  elbow  apparatus,  also  originated  by 
v.  Klobukow,  deserves  mention.  This  allows  the  gases  set 
free  at  the  electrodes  to  be  separately  collected,  and  accord- 
ingly permits  of  a  quantitative  examination  of  the  same.  The 
apparatus  is  readily  understood  from  Fig.  76.  The  corks, 

which  are  preferably  paraffined ,  carry 
thick  platinum  wires  to  which  the 
round  flat  plates  which  serve  as  elec- 
trodes are  welded  at  angles  of  45°. 
The  form  of  the  electrodes  is,  of 
course,  not  confined  to  any  particular 
one ;  v.  Klobukow  also  suggests 
round  fluted  platinum  foils,  spirally-wound  wire,  or  pointed 
electrodes. 

In  case  the  anode  and  cathode 
liquids  are  to  be  kept  separate  by 
a  porous  membrane,  v.  Klobukow 
proposes  the  arrangement  shown  in 
Fig.  77.  The  two  separate  arms 
have  close-fitting  ground  faces, 
which  are  cemented  into  a  brass 
mounting.  A  tight  joint  is  ob- 
tained by  a  hinge  and  screw. 

An  electrolytic  apparatus,  depending  upon  another  prin- 
ciple and  serving  other  purposes,  which  nevertheless  might 
be  useful  for  quantitative  work,  is  described  by  Hofer.* 
Fig.  78  shows  two  electrode  chambers  of  glass  provided  with 
inlet  and  outlet  tubes  for  the  electrolyte,  which  is  con- 


FIG  77. 


*  Ber.  deutsch.  chem.  Ges.,  27,  461. 


HISTORICAL. 


101 


FIG.  78. 


ducted  in  a  continuous  stream  through  the  apparatus.  There 
is  also  an  escape  tube  for  the 
gases  generated.  The  two  halves, 
between  which  parchment  paper 
or  other  porous  diaphragm  is  in- 
terposed, are  fastened  together  by 
means  of  a  firmly  cemented  con- 
nection provided  with  a  screw. 
The  electrodes  have  the  form  of 
spirals  of  platinum  wire,  0.8  mm. 
in  thickness,  or  of  small  platinum 
plates  attached  to  wires.  The  con- 
necting wires  pass  through  the  gas 
outlet  tubes,  and  in  case  the  gases  are  to  be  collected,  they 
are  carried  on  through  T  tubes  placed  at  the  top  and  made 
tight  with  rubber  stoppers. 

The  liquid  to  be  electrolyzed  is  contained  in  a  dropping- 
funnel,  the  tube  of  which  is  connected  by  rubber  tubing  to 
the  lower  inlet  tube  of  one  section  of  the  apparatus.  The 
liquid  is  thus  continuously  brought  to  the  particular  electrode 
and  is  made  to  circulate  through  the  cell  from  the  bottom  to 
the  top.  It  flows  out  through  the  outlet  tube,  thence  through 
a  piece  of  rubber  tubing  provided  with  a  screw  pinch-cock 
for  regulating  the  flow,  and  into  a  vessel  placed  at  a  lower 
level. 

This  piece  of  apparatus,  which  has  hitherto  been  used  only 
for  the  study  of  organic  decompositions,  might  perhaps  be 
suitable  for  the  quantitative  determination  of  gases. 

HISTORICAL. 

As  in  every  other  new  branch  of  science,  the  development 
of  electrolysis  has  been  purely  empirical.  From  a  great  number 
of  observations,  collected  with  diligence  and  perseverance,  in 


102         QUANTITATIVE  ANALYSIS   BY   ELECTROLYSIS. 

course  of  time  the  most  suitable  conditions  were  determined, 
while  the  nature  of  the  reactions  was  not  always  exactly  under- 
stood. It  was  reserved  to  the  most  recent  development  of 
electro-chemistry  to  throw  light  upon  the  important  factors 
of  quantitative  electrolysis,  and  to  make  clear  the  significa- 
tion and  value  of  the  current  magnitudes  and  other  con- 
ditions. 

The  first  researches  on  the  electrolytic  determination  of 
metals  were  of  a  purely  qualitative  nature.  Shortly  after  the 
discovery  of  the  decomposition  of  water  by  the  electric  cur- 
rent, Cruikshank  (1801)  brought  forward  the  conjecture, 
based  on  observations  of  copper  separation,  that  the  galvanic 
current  might  be  used  for  the  qualitative  determination  of 
metals.  Very  little  interest  was  taken  in  his  suggestion. 
Fischer  (1812)  identified  arsenic  by  electrolysis,  Cozzi  (1840) 
the  presence  of  metals  in  general  in  animal  fluids ;  also  Gaul- 
tier  de  Claubry  (1850)  recommended  the  employment  of  the 
current  for  discovering  poisonous  metals  in  mixtures  which 
contain  organic  substances. 

Charles  L.  Bloxam  (1860)  took  up  Gaultier's  researches 
and  worked  out  many  methods  which  attempted  to  make  pos- 
sible the  identification  of  arsenic  and  antimony  in  the  presence 
of  other  metals.  He  was  able  to  rely  to  a  certain  extent  upon 
the  directions  of  Morton  (1851)  for  the  separation  of  metals 
from  mixtures. 

It  had  already  been  observed  by  Becquerel  (1830)  that  lead 
and  manganese  often  separated,  not  as  metals  at  the  negative 
pole,  but  at  the  positive  pole  in  the  form  of  oxides,  a  fact 
which  permitted  of  the  ready  separation  of  these  metals  from 
others.  A  series  of  investigations  on  the  decomposition  of 
inorganic  metallic  salts  then  followed  by  Despretz  (1857),  by 
Nickles  (1862),  and  by  Wohler  (1868),  of  entirely  qualitative 
nature.  Likewise  the  summary  of  the  electro-chemical  in- 


HISTORICAL.  103 

vestigations  of  A.  C.  and  E.  Becquerel  only  gave  a  synopsis  of 
the  qualitative  electrolytic  reduction  of  the  metals. 

The  results  in  this  direction  were  accordingly  so  abundant, 
that,  based  upon  them,  quantitative  electrolysis  developed  with 
comparative  rapidity. 

The  field  of  quantitative  investigation  was  first  opened  by 
W.  Gibbs  (1864),  who  carried  out  an  investigation  on  the 
electrolytic  determination  of  copper  and  nickel,  which  included 
a  description  of  the  methods  for  the  determination  of  silver 
and  bismuth  in  the  form  of  metals,  as  well  as  of  lead  and 
manganese  in  the  form  of  peroxides.  He  also  published 
studies  on  the  separation  of  zinc,  nickel,  and  cobalt.  The 
possibility  of  the  quantitative  determination  of  copper  was 
confirmed  by  Luckow  (1865),  who  had  worked  at  it  for  a 
number  of  years.  The  quantitative  electrolytic  determination 
of  metals  was  entitled  by  him  "  electro- metal-analy sis. "  This 
author  published  at  the  same  time  a  series  of  directions  for 
the  method  of  using  the  current  for  analytical  work,  and  by 
these  precise  instructions  laid  the  foundation  for  many  later 
researches. 

The  attention  of  investigators  was  now  directed  principally 
towards  the  chemical  reactions  taking  place  in  the  cell  with 
the  use  of  different  sources  of  current  and  under  varying 
physical  conditions.  The  questions  as  to  the  suitable  salts  of 
the  metals  and  the  appropriate  solvents  to  be  used  and  the 
proper  substances  to  be  added  to  the  solutions  were  investi- 
gated and  determined.  Wrightson  (1876)  called  attention  to 
the  fact  that  the  presence  of  other  metals  influences  the  accu- 
racy of  copper  determinations,  and  ascertained  the  limits  for 
which  the  presence  of  antimony  in  copper  still  permitted  the 
accurate  determination  of  the  latter.  The  results  obtained 
with  cadmium,  zinc,  and  other  metals  were  as  yet  unsatisfac- 
tory. 


104        QUANTITATIVE   ANALYSIS   BY   ELECTROLYSIS. 

At  the  same  time  as  the  announcement  of  the  electrolytic 
determination  of  gallium  in  alkaline  solutions  by  Lecoq  de 
Boisbaudran  (1877),  the  first  investigation  of  Parodi  and 
Mascazzini  appeared  (1877).  In  the  latter,  notice  was  given 
of  the  determination  of  zinc  from  the  solution  of  its  sulphate, 
to  which  had  been  added  an  excess  of  ammonium  acetate. 
The  authors  also  found  it  possible  to  quantitatively  precipitate 
metallic  lead  from  an  alkaline  tartaric  acid  solution  containing 
an  alkali  acetate. 

To  Richert  (1878)  we  owe  the  first  accurate  directions 
concerning  tho  determination  of  manganese.  He  observed 
that  this  metal  may  be  completely  separated  from  solutions  of 
the  nitrate  in  the  form  of  an  oxide  at  the  positive  pole.  This 
characteristic  made  possible  the  electrolytic  separation  of 
manganese  from  other  metals,  as  copper,  cobalt,  nickel,  zinc, 
etc. 

Some  of  the  papers  appearing  at  the  same  time  by  Luckow, 
F.  W.  Clarke,  and  J.  B.  Haunay  treat  of  the  electrolytic 
determination  of  mercury,  which  readily  separates  from  solu- 
tions of  its  chlorides  or  solutions  of  inercurous  sulphate. 

F.  W.  Clarke  (1878)  succeeded  in  finding  a  method  for 
the  electrolytic  determination  of  cadmium  by  separating  the 
metal  from  solutions  of  its  acetate,  a  circumstance  which  Yver 
(1880)  employed  for  separating  cadmium  from  zinc.  Cadmium 
is  not  precipitated  in  the  presence  of  nitric  acid,  by  which  the 
same  author  succeeded  in  separating  this  metal  from  copper.* 

Beilstein  and  Jawein  (1879)  satisfactorily  employed  solu- 
tions of  the  double  cyanides  in  the  determination  of  zinc. 

According  to  Fresenius  and  Bergmann  (1880),  nickel  and 
cobalt  were  successfully  separated  from  solutions  which  con- 
tained an  excess  of  free  ammonia  and  ammonium  sulphate. 

*  This  method,  however,  is  uot  quantitative. 


HISTORICAL.  105 

Smith  (1880)  started  upon  the  first  of  his  series  of  investi- 
gations with  the  electrolysis  of  uranium  acetate,  which  allowed 
the  quantitative  separation  of  the  uranium  in  the  form  of  a 
hydrated  sesquioxide,  a  property  which  is  shared  by  molyb- 
denum in  solutions  of  ammonium  molybdate  containing  free 
ammonia.  *  Further  investig;  tio:is  of  this  well-known  chemist 
include  the  electrolysis  of  salts  of  tungsten,  vanadium,  and 
cerium,  and  more  recently  experiments  on  the  separation  of 
metals  from  potassium  cyanide  solutions,  f 

Luckow  (1880)  rendered  special  service  in  the  publication 
of  his  observations  on  the  reactions  which  take  place  during 
electrolysis  in  addition  to  the  reduction  of  the  metals.  He 
pointed  out  the  reduction  from  higher  states  of  oxidation 
to  lower  in  the  case  of  chromic  acid,  iron,  and  uranium  salts. 
He  showed,  on  the  other  hand,  that  sulphites  and  thiosulphates 
are  oxidized  to  sulphates.  Luckow  embodied  the  results  of 
his  observations  in  the  law,  that  in  general  the  action  of  the 
electric  current  on  acid  solutions  is  that  of  reduction,  on 
alkaline  solutions  that  of  oxidation. 

A.  Classen  and  his  students,  in  the  year  1881,  began  in- 
vestigations on  quantitative  analysis  by  electrolysis,  which 
embraced  the  observation  of  nearly  all  the  metals.  Classen 
first  pointed  out  the  value  of  oxalic  acid  and  of  the  oxalates 
in  the  form  of  double  salts  with  metals  (1881).;):  He  also 


*  According  to  the  investigations  undertaken  by  M.  Heidenreich  (Ber. 
deutsch. -chera.  Gesell.,  29,  1587)  this  method  gives  unsatisfactory  results. 

f  Compare  experiments  given  in  the  special  part. 

\  Smith,  in  his  Electrochemical  Analysis  (p.  44),  states  that  Parodi  and 
Mascazzini  (Gazetta  chim.  it.,  vol.  8,  p.  178),  in  the  year  1879,  therefore 
two  years  before  the  appearance  of  the  author's  publication,  had  already 
announced  that  iron  and  antimony  could  be  separated  in  a  dense  form,  if 
solutions  of  the  sulpho-salts  of  antimony  and  chloride  of  iron  containing 
acid  ammonium  oxalate  were  electrolyzed.  The  article  by  Parodi  and  Mas- 
cazzini referred  to  states  as  follows:  "  Questi  due  metalli  si  depongono 


106        QUANTITATIVE  ANALYSIS   BY   ELECTROLYSIS. 

worked  out  a  large  number  of  electrolytic  methods,  the  details 
of  which  will  be  given  under  the  corresponding  metals. 

At  about  the  same  time  Reinhardt  and  Ihle  recommended 
the  oxalic  acid  double  salts  for  the  determination  of  zinc. 

Gibbs  attempted  (1880),  by  the  use  of  a  mercury  cathode, 
to  determine  the  metals  through  the  increase  in  weight  of  the 
mercury,  a  method  which  Luckow  (1886)  applied  also  to  the 
analysis  of  zinc. 

Since  the  year  1886  a  great  number  of  publications  have 
annually  appeared,  the  enumeration  of  which  would  require 
too  great  space. 

Nevertheless  the  experiments  of  Vortmann  (1894)  on  the 
electrolytic  determination  of  the  halogens  by  the  employment 
of  a  silver  cathode  are  worthy  of  mention.  The  silver  halides 
adhere  firmly  to  the  electrode,  the  increase  in  weight  of  which 
gives  directly  the  quantity  of  halogen  which  has  separated. 

Greater  stress  was  laid  on  the  physical  relations  in  the- 
investigations  which  originated  exclusively  from  the  observa- 
tions of  Kiliani  (1883)  on  the  significance  of  the  tension  in 
electrolytic  experiments.  Le  Blanc  was  the  first  to  determine 
the  decomposition  tension  values  for  solutions  of  various  metals, 
and  through  his  work  laid  the  foundation  based  upon  which 
Freudenberg  (1891)  conducted  a  separation  of  several  metals 
by  a  mere  variation  of  the  tension. 

sotto  forma  elementare  compatti  e  perfettamente  aderenti  sul  polo  negative 
in  platino  e  cive  :  1'antimonio  dal  chloruro  diluito  nel  tartrato  ammonico 
busico  ed  auche  dalle  dissoluzioni  dei  solfosoli ;  il  ferro  dal  sesquiossido 
disciolto  nel  1'ossalato  acido  di  amtnouiaca."  It  is  therefore  evident  that 
the  authors  used  tartaricacid  solutions  for  antimony.  Tlie  author  (Classen) 
first  proposed  the  use  of  ammonium  oxalate  in  the  detenu  nation  of  iron. 
The  acid  ammonium  oxalate  employed  for  this  purpose  by  Parodi  and 
Mascazzini  is  not  at  all  suited  for  the  determination  of  iron,  since,  in  the 
first  place,  it  forms  no  soluble  ferrous  double  salts,  and,  in  the  second  place, 
from  solutions  of  the  ferric  double  salts  not  even  a  trace  of  iron  is  precipi- 
tated until  the  acid  ammonium  oxalate  has  been  decomposed  by  the  current. 


ARRANGEMENTS   FOR  ANALYSIS.  107 

As  the  methods  of  electrolysis  were  gradually  developed, 
apparatus  suitable  to  the  special  requirements  were  con- 
structed. The  instruments  and  arrangements  for  quantitative 
electrolysis  at  present  in  general  use  have  originated  in  the 
Aachen  laboratory,  where,  moreover,  both  dynamos  and  ac- 
cumulators were  for  the  first  time  employed  as  sources  of 
current. 

ARRANGEMENTS   FOR  ANALYSIS. 

The  question  as  to  the  most  practical  equipment  for  elec- 
trolytic research  does  not  permit  of  a  general  answer,  owing 
to  the  many  details,  such  as  the  construction  of  the  "building, 
the  arrangement  of  rooms,  etc.,  upon  which  it  depends. 
After  solving  the  problem  as  to  the  most  serviceable  source 
of  current,  and  deciding  in  favor  of  accumulators  in  combina- 
tion with  a  dynamo  or  thermopile,  the  details  of  the  equip- 
ment can  only  be  described  from  a  certain  point  of  view, 
according  to  the  requirements  which  must  be  fulfilled.  The 
laboratory  at  Aachen  has  followed  the  development  of  quan- 
titative electrolysis,  and  beginning  with  the  smallest  and 
simplest  equipment  has  gradually  attained  a  most  elaborate 
one.  Three  equipments  may  therefore  be  profitably  de- 
scribed; first,  the  simplest  and  most  useful  arrangement  for 
small  requirements;  second,  the  former;  and  third,  the  pres- 
ent electrolytic  outfit  of  the  Technical  High  School  at  Aachen. 

Kriiger  *  has  published  a  general  review  of  the  equipment 
of  electrolytic  laboratories,  which  contains  many  valuable  sug- 
gestions, the  repetition  of  which,  however,  would  occupy  too 
great  a  space.  The  choice  of  special  apparatus  depends  so 
much  upon  the  individual  taste  that  exact  directions  are 


*  Elektrochem.  Ztsckr.,  2,  pp.  73,  104,  129,  174,  207,  251  ;  3,  7,  76,  129. 


108        QUANTITATIVE   ANALYSIS   BY    ELECTROLYSIS. 

practically  impossible.  Indeed  Kr tiger  warmly  recommends 
a  set  of  instruments  the  practical  advantages  of  which  have 
not  been  confirmed  in  the  Aachen  laboratory. 


ARRANGEMENT   FOR   SMALLER   EXPERIMENTS. 

The  equipment  needed  for  carrying  out  a  single  electro- 
lytic experiment  when  a  constant  source  of  current  is  at  hand 
is  an  extremely  simple  one. 

A  standard,  with  a  dish  and  electrode,  and  instruments  for 
measuring  the  tension  and  current  strength,  are  all  that  are  re- 
quired. Since,  however,  it  is  often  desirable  to  conduct  several 
experiments  simultaneously  on  a  small  scale,  an  arrangement 
will  be  described  which  has  the  advantage  that  it  may  be  con- 
structed by  every  one  who  makes  use  of  electrolysis. 

The  chief  requirement  is  that  it  shall  be  possible  to  con- 
stantly observe  the  tension  and  strength  of  the  current.  This 
may  be  accomplished  with  the  use  of  one  amperemeter  and 
one  voltmeter  for  any  number  of  experiments,  in  the  follow- 
ing manner : 

A  wooden  block,  /(Fig.  79),  is  placed  before  the  binding- 
post  to  which  the  negative  pole  of  the  source  of  current  is  at- 


FIG.  79. 


tached.     This  block  has  a  number  of  holes  bored  in  its  upper 
side,  and  into  these  holes  are  set  inverted  thimbles  filled  with 


ARRANGEMENT   FOR   SMALLER    EXPERIMENTS.      109 

mercury.     These  mercury-cups  are  arranged  so  that  there  are 
five  in  a  row  along  one  edge  and  four  along  ; 
the  other.     (Fig.  80..) 


<&>     <r>    <=>    <=> 


A  resistance-box  for  the  regulation  of  the 
current  is  required  for  each  separate  experi- 
ment.     The  description  will  be  limited  to  four  simultaneous 
electrolyses. 

The  rheostats  w^  w^  wt,  w41  of  the  form  designated  in  the 
sketch,  are  then  placed  in  front  of  the  block  7,  and  a  second 
block,  77,  having  four  mercury-cups  on  one  side  and  one  on 
the  other,  is  added.  An  amperemeter  and  a  voltmeter  com- 
plete the  outfit. 

The  connections  are  made  as  follows  (Fig.  79) :  Four  short 
wires  extend  from  the  negative  pole  into  the  mercury-cups  1, 
2,  3,  4,  of  board  /,  the  fifth  cup  of  which,  #,  is  connected 
directly  with  the  amperemeter.  The  second  binding-screw 
of  the  amperemeter  is  connected  by  a  wire  to  the  negative 
pole.  Four  short  wires  lead  from  the  corresponding  mercury- 
cups  of  board  7,  1',  2',  3',  4',  to  the  resistance- boxes  w^  w» 
w»  w4,  the  other  binding-posts  of  which  are  connected  both 
with  the  electrolytic  cells  and  also  with  the  mercury-cups  sl9 
*•>  *8?  *45  of  board  77,  in  the  corresponding  manner.  The  mer- 
cury cup  s,  situated  by  itself  on  board  77,  opposite  the  four 
mercury-cups  just  mentioned,  is  connected  with  the  instru- 
ment for  measuring  the  tension ;  and  the  second  binding-post 
of  the  latter  is  connected  by  a  wire  to  the  positive  pole.  In 
order  to  complete  the  circuit  it  is  only  necessary  to  connect  the 
cups  1-1',  2-2',  3-3',  4-4'  by  short  bent  wires.  For  the 
cell  1,  for  example,  beginning  at  the  negative  pole,  the  cur- 
rent pursues  the  following  path:  Negative  pole,  1—1',  «0,, 
cell  1,  positive  pole.  The  current  travels  likewise  in  the 
other  experiments.  In  order  at  the  same  time  to  measure  the 
current  strength,  the  wire  connection  is  laid  from  a  to  1'  (2', 


110         QUANTITATIVE   ANALYSIS   BY   ELECTROLYSIS. 

3',  4/)  and  the  connection  1-1 '  is  broken  (correspondingly  2-2', 
3-3',  4-4/).  By  this  arrangement  the  current,  in  passing 
from  the  negative  pole,  is  forced  to  flow  through  the  ampere- 
meter, thence  over  a-V  to  the  rheostat,  etc.  In  order  that 
the  correct  value  for  the  current  strength  may  be  obtained  by 
this  operation,  the  wires  which  make  the  connections  1-1', 
2-2',  etc.,  must  have  a  resistance  equal  to  that  of  the  am. 
peremeter,  while  the  connections  a-V ',  #-2',  etc.,  must  be 
made  with  wires  having  practically  no  resistance.  (For  fur- 
ther details  see  p.  122.)  ' 

From  the  electrolytic  cells  the  circuit  is  completed,  on 
the  one  hand  by  wires  having  extremely  low  resistances  run- 
ning to  the  positive  pole,  and  on  the  other  hand  by  similar 
wires  connected  to  the  binding-posts  of  the  resistance -boxes, 
or  what  amounts  to  the  same  thing,  to  the  cups  $1?  s2,  s3,  s4. 
Between  one  of  the  latter  and  the  positive  pole  the  tension 
must  be  measured,  since  here  the  fall  in  tension  is  due  to  the 
resistance  of  the  cell  only.  It  is  accordingly  sufficient  to 
make  the  connection  s1  —  s,  s2  —  s,  etc.,  in  order  to  imme- 
diately obtain  the  difference  of  potential  in  the  corresponding 
cell.  The  'simultaneous  measurement  of  several  cells  is  of 
course  out  of  the  question. 

This  simple  appliance,  the  principles  of  which  recur  in  the 
following  descriptions,  can  be  prepared  by  anyone  from  the 
simplest  materials,  so  that,  as  already  stated,  it  is  very  suit- 
able for  students,  since  by  working  with  it  they  become  ac- 
quainted with  the  methods  of  making  connections  and  the 
manipulation  of  more  elaborate  apparatus. 

The  resistance- boxes  permit  of  a  variation  of  the  tension 
and  current  strength  sufficient  for  most  purposes. 


THE    ELECTRO-CHEMICAL   INSTITUTE  AT   AACHEN.   Ill 

FORMER    EQUIPMENT  OF    THE    ELECTRO-CHEMICAL 
INSTITUTE  AT  AACHEN. 

This  system  is  based  upon  the  employment  of  a  dynamo 
of  the  type  described  on  page  62,  the  current  from  which  can 
be  employed  both  directly  and  for  charging  accumulators. 

As  already  stated,  the  machine  described  has  a  tension  of 
10  volts  with  1,000  revolutions.  The  tension,  while  the 
machine  is  in  use,  is  measured  by  a  galvanometer  or  other 
instrument  which  shows  the  tension  directly.  In  Fig.  81,  the 
tension  indicator  marked  G  is  connected  with  both  ends  of 
the  brass  resistance  MMr 

Siemens  &  Halske  describe  the  tension  indicator  as 
follows :  It  consists  of  an  electro-magnet,  beside  one  pole  of 
which  stands  on  edge  a  piece  of  iron  which  has  the  same 
polarity,  and  is  therefore  repelled  by  it  in  proportion  to  the 
strength  of  the  magnetism,  and  so  of  the  electric  current 
which  passes  around  the  instrument.  The  extent  of  the 
repulsion  is  measured  on  a  scale  on  which  plays  an  index 
attached  to  the  piece  of  iron  which  is  repelled.  The  indica- 
tions of  the  instrument  are  not  entirely  independent  of  the 
residual  magnetism  ;  the  direction  of  the  current  in  the 
instrument  must  therefore  be  alwaj^s  the  same.  This  result- 
is  accomplished  by  a  small  adjustable  permanent  magnet  in 
front  of  the  lower  pole  of  the  electro-magnet ;  this  shows  the 
direction  of  the  current  in  the  instrument,  and  stops  the 
index  if  the  current  is  in  the  wrong  direction.  (See  Z, 
Fig.  81.)  If  this  occurs,  the  wires  leading  the  current  to 
the  instrument  must  be  interchanged. 

The  instrument  is  supplied  with  a  brass  ring,  which, 
before  the  current  passes,  is  placed  on  the  round  weight  of 
the  index,  and  must  then  turn  the  index  to  the  zero  point. 
If  this  is  not  the  case,  the  instrument  is  not  plumb.  When 
the  instrument  is  in  use,  the  ring  is  removed. 


112         QUANTITATIVE    ANALYSIS   BY   ELECTROLYSIS. 

As  already  stated,  the  laboratory  apparatus  constructed 
by  Siemens  &  Halske  is  capable  of  carrying  on,  at  the  same 
time,  a  large  number  of  electrolytic  determinations  on  the 


small  scale,  requiring  currents,  differing  in  strength  and  ten- 
sion, so  that  each  determination  is  independent  of  the  rest. 
According  to  the  description  of  Siemens  &  Halske,  this 


THE    ELECTROCHEMICAL   INSTITUTE  AT   AACHEN.    113 

result  is  obtained  essentially  by  passing  far  the  greater  part 
of  the  current  through  a  brass  wire-gauze  resistance,*  the 
individual  determinations  being  made  by  small  branch  cur- 
rents which  may  be  independently  varied  in  intensity  by 
attaching  their  conductors  to  different  portions  of  the  wire- 
gauze  resistance. 

The  dynamo  machine  is  connected  by  short  heavy  con- 
ductors to  the  ends  M  Mj  of  the  zigzag  brass  wire-gauze 
resistance.  These  ends  of  the  resistance  are  also  connected, 
by  smaller  wires,  with  the  instrument  which  shows  directly 
the  tension  at  the  resistance.  Care  must  be  taken  that  this 
instrument  always  shows  the  same  tension,  i.e.,  that  the 
velocity  of  the  machine  is  uniform.  If  the  tension  at  the 
ends  of  the  resistance  is  6  volts,  and  the  resistance  is  made 
up  of  24  equal  parts,  the  ends  of  which  are  connected  with 
binding-screws,  the  difference  in  tension  between  any  two 
adjacent  binding-screws  is  ^  =  ^  volt.  If  the  tension  at 
the  first  screw  is  0,  the  tensions  at  the  following  screws  are 
i>  f  i  f  >  1?  f  >  etc.,  volts ;  that  is,  the  whole  interval  of  6  volts 
is  divided  into  portions  of  J  volt  each. 

If,  now,  a  current,  small  in  proportion  to  the  current 
passing  through  the  resistance,  is  taken  out  between  any  two 
binding-screws  for  an  electrolytic  determination,  the  tension 
between  the  screws  is  not  materially  changed  ;  the  wires 
carrying  this  current  can  be  connected  with  any  binding 
screws  without  any  change  in  the  main  current ;  moreover, 

*  When  the  same  source  of  current  is  used  for  carrying  on  a  number 
of  dissimilar  experiments  simultaneously,  the  employment  of  resistances 
and  the  loss  of  a  part  of  the  energy  is  unavoidable.  The  apparatus  con- 
structed by  Siemens  and  Halske  has  the  advantage  that  it  makes  use  of 
only  a  single  resistance,  while  with  all  other  arrangements  as  many  sepa- 
rate resistances  are  required  as  experiments  are  conducted. 


114         QUANTITATIVE  ANALYSIS   BY    ELECTROLYSIS. 

the  introduction  of  a  number  of  such  currents  does  not 
materially  change  the  tension,  and  the  tension  for  any  given 
determination  can  be  varied  at  will  without  affecting  the 
others, 

In  the  apparatus  used  by  the  author,  Fig.  81  (one- 
twentieth  natural  size),  the  brass  wire  gauze  resistance  is 
divided  into  20  equal  parts  marked  1,  2,  3,  etc.  As  already 
stated,  the  machine,  at  15000  revolutions,  has  a  current- 
strength  of  60  amperes  and  a  tension  of  10  volts.  Of  the  60 
amperes,  40  are  conducted  through  the  resistance,  so  that  20 
remain  for  electrolytic  determinations. 

The  difference  of  tension  between  two  adjacent  binding- 
screws  is  J§  =  £  volt.  The  tension,  that  is,  at  the  screw 
marked  19,  is  £  volt,  at  18  =  1,  at  17  =  1J,  at  16  =  2,  at 
0  =  10  volts. 

The  current  from  the  machine  enters  by  a  heavy  copper 
conductor  at  the  screw  marked  0,  and  passes  out  at  that 
marked  20. 

On  the  board  BBBB  are  fastened  6  T-shaped  galvanized- 
iron  strips,  Sx,  S2,  S3,  S4,  S5,  S6,  six  resistances  of  0.1  ohm 
each,  Wx,  W2,  W3,  W4,  W5,  W6  (to  allow  the  strength  of 
current  in  single  experiments  to  be  measured),  and  the  brass 
strip  M2.  S1  is  connected  by  a  wire  with  Wx,  S2  with  W2,  S3 
with  W3,'  S4  with  W4,  S5  with  W5,  and  S6  with  W6.  The  iron 
strips  may  be  connected  with  the  binding-screws  1,  2,  3,  etc., 
by  means  of  wires  and  the  brass  screws  K1?  K2,  etc.  If  the 
apparatus  is  used  as  shown  in  the  cut,  and  1,  2,  or  3  is 
connected  with  S1?  4,  5,  or  6  with  S2,  7,  8,  or  9  with  S3,  10, 
11,  or  12  with  S4,  13,  14,  15,  or  16  with  S5,  and  one  of  the 
others  with  S6,  the  strongest  current  is  at  Wv  and  the  weakest 
at  W6.  Any  strip  may,  of  course,  be  connected  with  any 
binding-screw. 

In  performing  electrolysis,  the  solutions  to  be  acted  on 


THE   ELECTRO-CHEMICAL   INSTITUTE  AT  AACHEN.   115 

are  placed  in  connection  with  a  negative  pole  nv  nv  or 
w3,  etc.  (on  the  resistances  Wv  W2,  or  W3,  etc.),  and  a 
positive  pole  pv  p^  or  p%,  etc.,  on  the  brass  strip  M2,  the 
connections  being  made  according  to  the  strength  of  current 
desired. 

Moreover,  as  shown  by  the  examples  given  later,  several 
determinations  requiring  the  same  strength  of  current  may 
be  connected  with  any  pair  of  poles,  n^  and  pv  n2  and  p^,  etc. 
In  order  to  connect  more  conveniently  with  the  platinum 
dishes  containing  the  solutions  for  electrolysis,  n^  and  pv  for 
instance,  may  be  connected  with  a  brass  strip  Z  (the  con- 
nection with  n^  only  is  shown  in  the  cut),  to  which  are 
attached  a  number  of  binding-screws,  zv  22,  etc. 

The  tension  and  the  strength  of  the  current  may  be 
measured  at  each  dish.  For  example,  if  the  tension  at  the 
dish  connected  with  W2  is  to  be  measured,  the  plugs  from 
the  galvanometer  are  inserted  at  52  and  c2 ;  if  they  are 
inserted  at  «2  and  52,  the  tension  in  the  resistance  is  meas^ 
ured,  which,  multiplied  by  10,  gives,  in  amperes,  the  strength 
of  the  current  acting  on  the  solution  connected  with  W2. 

In  order  to  test  the  working  of  the  apparatus,  the  tension 
at  the  divisions  of  the  wire-gauze  resistance  was  directly 
measured  by  a  torsion  galvanometer,  with  the  following 
results : 


116        QUANTITATIVE  ANALYSIS   BY    ELECTROLYSIS. 


Resistance  marked 

Connected  with  Binding 
Screw,  marked 

Tension  in  Volts. 

w, 

1 

10.300 

w, 

2 

9.900 

w, 

3 

9.400 

W2 

4 

8.950 

W2 

5 

8.300 

wa 

6 

7.750 

W8 

7 

7.200 

W8 

8 

6.650 

w$ 

9 

5.950 

W4 

10 

5.500 

W4 

11 

5.050 

w. 

12 

4.500 

W6 

13 

4.000 

W6 

14 

3.450 

W6 

15 

2.850 

W6 

16 

2.300 

W6 

17 

1.700 

W6 

18 

1.100 

w. 

19 

0.560 

W6 

20 

0.007 

For  the  measurement  of  the  strength  of  the  current  at 
the  screws  1  to  20,  a  cell  was  used  which  had  a  copper  elec- 


THE   ELECTRO-CHEMICAL   INSTITUTE  AT   AACHEN.    117 

trode,*  and  contained  150  cc.  of  a  15  per  cent  solution  of 
copper  sulphate ;  this  cell  was  connected  to  the  resistance  W6 
(binding-screws,  ne  and  pQ).  The  screws  1  to  20  were  then 
successively  connected  with  the  bar  S6,  and  the  deflection  of 
the  galvanometer  read,  the  plugs  connecting  it  being  placed 
in  #6  and  b6.  After  this  reading,  the  tension  in  the  cell  was 
read,  for  each  screw  connection,  by  placing  the  plugs  in  £>6 
and  cQ. 

In  order  to  control  the  rate  of  the  machine  during  the 
-experiment,  pl  and  nl  on  the  resistance  W1  were  connected 
through  a  rheostat ;  and  the  tension  at  the  binding-screw  1 
(connected  with  Sj)  was  determined  by  a  second  torsion 
galvanometer,  the  plugs  from  which  were  inserted  at  b^ 
and  cr 

The  results  of  these  experiments  are  given  in  the  following 
table  in  the  columns  included  under  I. 

A  second  series  of  experiments  was  conducted  to  deter- 
mine the  strength  of  the  current  by  the  quantity  of  copper 
precipitated. 

Six  platinum  dishes,  as  nearly  alike  as  possible,  were 
filled  with  150  cc  each  of  a  15  per  cent  solution  of  copper 
sulphate,  supplied  with  copper  eiectrodes  (see  note  below), 
and  different  quantities  of  copper  precipitated  in  the  same 
time.  These  experiments  were  conducted  in  three  series,  as 
follows :  — 

Series  1.  I.,  IV.,  VIII.,  XII.,  XVI.,  XIX. 
Series  2.  II.,  V.,  IX.,  XIII.,  XVII.,  XX. 
Series  3.  III.,  VI.,  X.,  XL,  XIV.,  XVIII. 


*  The  cell  consisted  of  a  platinum  dish,  and  the  positive  electrode  was  a 
round  piece  of  sheet-copper  (of  the  form  of  the  platinum  electrode  shown  in 
Fig.  55),  6  cm.  in  diameter  and  2  mm.  thick.  The  electrodes  were  2.5  cm. 
apart. 


118        QUANTITATIVE  ANALYSIS   BY   ELECTROLYSIS. 

Of  the  columns  included  under  II.,  A  gives  the  strength 
of  the  current  as  determined  from  the  precipitated  copper ; 
B,  the  results,  in  a  few  cases,  of  the  measurement  of  the 
strength  of  current  by  a  torsion  galvanometer  ;  and  C, 
the  tension  measured  at  the  same  time  with  the  torsion 
galvanometer. 


Binding 
Screw. 

I. 

II. 

Amperes. 

Volts, 
Experi- 
ment. 

Volts, 
Machine. 

A, 
Amperes. 

B,  Am- 
peres. 

c, 

Volts. 

I. 
II. 

18.018 
15.352 

7.900 
7.400 

10.90 
9.90 

15.97 
14.04 

9.200 
9.000 

III. 

13.231 

7.100 

10.10 

10.86 

10.800 

7.900 

IV. 

11.615 

6.650 

10.10 

8.87 

- 

7.400 

V. 

10.302 

6.350 

10.30 

8.00 

- 

7.100 

VI. 

9.595 

6.010 

10.40 

6.04 

- 

5.500 

VII. 
VIII. 

8.383 
6,565 

5.710 
5.300 

10.50 
10.60 

4.97 

_ 

5.000 

IX. 

5.757 

5.100 

10.60 

4.21 

3.800 

4.500 

X. 

4.747 

4.700 

11.1-0 

4.03 

-       3.800 

XI. 

4.040 

4.250 

10.90 

3.75 

3.700 

3.100 

XII. 
XIII. 

3.838 
3.535 

3.800 
3.400 

11.00 
10.90 

3.54 
3.47 

: 

2.900 
2.500 

XIV. 

3.030 

2.850 

10.90 

3.09 

2.700 

2.300 

XV. 

2.520 

2.400 

11.05 

- 

- 

- 

XVI. 

2.120 

1.900 

11.00 

1.85 

- 

1.200 

XVII. 

1.560 

1.500 

11.00 

1.35 

- 

1.050 

XVIII. 

0.759 

0.890 

10.90 

0.76 

0.605 

0.600 

XIX. 

0.396 

0.290 

11.00 

0.54 

- 

0.360 

XX. 

0.000 

•.—  —  —  ^— 

0.007 

11.10 

—  ^^«—  «—  ^—  ^ 

0.00 



— 

0.007 

•         •   i  ^—~m 

THE  ELECTRO-CHEMICAL   INSTITUTE   AT  AACHEN.   119 

The  following  sixteen  experiments  were  made  simulta- 
neously under  the  same  conditions  as  before.  The  numbers 
in  column  A  express  the  quantities  of  copper  precipitated  in 
6.5  minutes ;  those  under  B,  the  tensions  measured  with  the 
torsion  galvanometer. 


A,  Copper. 

B,  Volts. 

f     0.7616    g       ] 

Binding  screw  I.  to  Wl 

1     0.7415    " 
I     0.8286    " 

7.10 

Screw  IV.  to  W2      .... 

(    0.6021    " 
0.5716    " 

5.30 

1     0.4788    "        J 

0.4155    " 

Screw  VIII.  to  W3  .     .     .     . 

<      0.3510    " 

3.30 

.     0.3535    " 

C.2648    « 

Screw  XII.  to  W4    .... 

<     0.2963    " 

1.80 

.     0.2652    " 

Screw  XVI.  to  W5  .... 

j     0.1435    "        | 
(     0.1470    "        f 

0.90 

Screw  XIX.  to  W6  .     .     .     . 

(     0.0363    "        ) 
(    0.0260    4t       ) 

0.23 

In  order  to  reach  a  conclusion  as  to  the  value  of  the 
apparatus  for  the  purposes  of  quantitative  analysis,  twelve 
determinations  were  carried  on  simultaneously,  at  the  author's 
request,  by  Dr.  Kobert  Ludwig,  formerly  assistant  in  the  In- 
organic Laboratory.  The  solutions  used  for  these  experi- 
ments were  of  iron,  cobalt,  tin,  antimony,  and  copper,  metals 


120        QUANTITATIVE  ANALYSIS   BY    ELECTROLYSIS. 


which,  as  will  be  shown  later,  require  currents  of  widely 
different  strengths  for  their  separation.  The  results  of  one 
series  of  these  experiments  are  subjoined. 


Taken. 

Found. 

I. 

0.3546    g 

Fe2O3 

0.2479 

g    Fe  =    0.3541    g    Fe2O3 

II. 

0.3836    " 

Fe203 

0.2691 

"    Fe  =    0.3844    "    Fe2O3 

III. 

0.2624    " 

Co 

0.2619 

"    Co 

IY. 

0.2234    " 

Co 

0.2231 

"    Co 

V. 

0.1145    " 

Sn 

0.1142 

"    Sn 

VI. 

0.2290    " 

Sn 

0.2290 

"    Sn 

VII. 

0.2025    " 

Sb2S3 

0.1444 

"    Sb  =    0.2019    "    Sb2S3 

VIII. 

0.1890    " 

Sb2S3 

0.1348 

"    Sb  =    0.1885    "    Sb2S3 

IX. 

0.1670    " 

Sb2S3 

0.1189 

"    Sb  =    0.1663    "    Sb2S3 

X. 

0.8374    " 

CuSO4 

0.2133 

"    Cu  =  25.47  %  Cu 

XI. 

0.8768    " 

CuSO4 

0.2225 

"    Cu  =  25.31  %  Cu 

XII. 

0.7905    " 

CuSO4 

(  0.1991 

"    Cu  =  25.29  %  Cu 

I 

Calculated  25.39  %  Cu 

In  general  the  current  of  the  dynamo  was  not  directly 
employed,  bu{;  was  used  to  charge  four  accumulators,  which 
sent  their  current  of  8  volts  to  the  electrolytic  work-bench. 
The  current  thus  transformed  was  employed  in  the  following 
manner  (Table  I). 

The  connection  of  the  cells  with  the  positive  conductor, 
which  carries  the  current  of  the  four  accumulators  to  the 
electrolytic  table,  is  effected  by  means  of  six  binding-screws 
(marked  1,  2,  3,  4,  5,  6).  For  connecting  the  cells  with  the 
negative  pole  of  the  source  of  current,  wooden  blocks  bearing 
separate  binding-posts  and  mercury-cups  (in  the  diagram  6) 
are  made  use  of.  The  arrangement  of  such  a  board  is  shown 
in  Fig.  82  (f  actual  size). 

The  cups  marked  1,  2,  3,  4  are  connected  with  the  four 
binding-posts  JT,  cups  5  and  6  with  the  negative  conductor 


THE  ELECTRO-CHEMICAL  INSTITUTE  AT  AACHEN.   121 


from  the  source  of  current,  and  cup  7  with  one  conductor 
from  the  amperemeter.  The  connections  between  the  cups 
are  made  by  plain  copper  forks  while  the  current  is  being 
measured,  and  otherwise  by  forks  upon  which  resistances 


0  O 


K 

1-7 


Binding  posts. 

Mercury  cups. 

Cup  7  is  connected  through 

the  measuring  circuit  with 

the  amperemeter. 


0    O 


FIG.  82. 

equal  to  the  resistance  of  the  measuring  instrument  are  rolled. 
A  fork  of  this  description  (resistance-roll)  is  shown  separately 
in  Fig.  83.  An  instrument  made  by  Hartmann  &  Braun 
(Bockenheim -Frankfurt  a.  M.)  serves  for  measuring  the  cur- 
rent strength.  This  instrument,  especially  constructed  for 
the  laboratory  of  instruction,  is  provided  with  two  scales  and 
pointers  (one  on  fech  side),  which  allow  of  its  being  observed 
from  all  points  on  the  work-bench.  The  pointer  of  the 


122        QUANTITATIVE  ANALYSIS   BY    ELECTROLYSIS. 

amperemeter  moves  over  a  scale  having  a  radius  of  16  cm. 
The  instrument  permits  of  the  measurement  of  currents  up  to 
2  amperes  in  decimals  of  0.05  ampere.  By  the  use  of  a 
resistance  which  may  be  connected  in  shunt,  the  range  of 
measurement  is  increased  tenfold.  The  resistance  of  the 
instrument  itself  is  0.32  ohm.  An  exactly  equal  resistance  is 
contained  in  the  resistance-roll. 

The  measurement  of  the  current  strength  in  the  cell  is 
conducted  as  follows :  A  moderately  high  resistance  of,  say, 
60  ohms  is  'inserted  in  the  circuit  of  the  connected  rheostat 
and  a  wire  from  a  positive  binding-screw  is  connected  with 


Resistance-roll    having   a   resistance 
equal  to  that  of  the  amperemeter. 


FIG.  83. 

the  anode  of  the  cell.  A  wire  running  to  one  of  the  neg- 
ative binding- screws  (for  example,  5,  Table  I,  Fig.  1)  i& 
now  attached  through  the  rheostat  to  the  cathode.  The 
arrangement  is  seen  from  Table  I,  Fig.  1.  It  only  remains 
to  connect  the  mercury-cups  4  and  7  (the  latter  connected 
with  the  amperemeter)  by  means  of  a  copper  fork.  With  a 
resistance  of  60  ohms  in  the  circuit  the  amperemeter  will 
show  only  a  small  deflection,  which  may  be  increased  to  the 
required  value  by  reducing  the  resistance  in  the  rheostat. 
This  having  been  done,  a  resistance- roll  is  inserted  between  the 
cups  4  and  6  and  the  copper  fork  between  4  and  7  is  removed, 
which  breaks  the  connection  with  the  amperemeter.  Since 
the  resistance  of  the  amperemeter  and  roll  are  equal,  a  current 


1-6  POSITIVE  BINDING  POS1 
.  SEPARATE  NEGATIVE  B 
0  MERCURY  CUPS. 
.  POSITIVE  CONDUCTORS 
NEGATIVE  CONDUCTORS 
I  CIRCUIT  CONNECTED  W 
\  MEASURING  INSTRUMEN 

PLAN  OF  WORK-BENCH  1 

tf 

1 

00 

p  0         °' 

j         1 

+fTo 

TABLE  I 
[NG  ARRANGEMENT  FOR  MEASURING  THE  CURRENT 
THE  USE  OF  A  SINGLE  J 
FIG.  1. 

r^ 

H          !" 

!^     °' 

I 

O                      1  —  5*  —  L  c            *°        O 

>        I^Ptf&f 

E  /p     i        cor 

>                     1                          3)                              -t- 

,         >     1 

.,., 

CONNECTION  FOR  MEASUREMENT 
CABLE  FROM  THE  DYNAMO  WITH  T(J£  AMPER^METER 
OR  ACCUMULATORS  j  1 
WITH  THE  RESISTANCE  \ 

% 

ELECTROLYTIC  CELL 
CELL 

BORATORY  OF  THE  ROYAL  TECHNICAL  HIGH  SCHOOL  AT  AACHEN. 

i  4-j 
1 

O   li 

ENGTH  OF  EACH  SEPARATE  ELECTROLYSIS  WIT 
REMETER. 

^J' 

_tF 

;y: 

—  »l 

OF  THE 

UNIVERSITY 


THE   ELECTRO-CHEMICAL   INSTITUTE   AT   AACHEN.    128 

corresponding  to  the  one  measured  flows  through  the  cell.  To 
observe  the  current  strength  during  the  electrolysis,  the  copper 
fork  is  placed  in  the  cups  4  and  7,  and  the  resistance- roll  is 
removed.  It  is,  therefore,  possible  to  measure  the  current 
strength  at  any  time  without  interrupting  the  current  through 
the  cell.  As  is  evident  from  Table  I,  Fig.  1,  the  construc- 
tion of  the  electrolytic  table  allows  24  separate  electrolyses  to 
be  conducted  simultaneously.  If  at  least  four  accumulators, 
with  a  tension  of  8  volts,  are  used,  a  considerable  number 
of  experiments  may  be  carried  out  at  the  same  time  quite  in- 
dependently of  one  another. 

The  former  equipment  of  the  private  laboratory  is,  with- 
out further  comment,  evident  from  Table  II.  It  includes  a 
special  connection  for  using  the  current  from  the  dynamo 
directly,  as  well  as  for  working  with  the  8  accumulators. 
The  wire-gauze  resistance,  described  on  p.  113,  serves  to 
reduce  the  current  when  charging  the  accumulators,  or  in  the 
direct  employment  of  the  same.  The  conductors  from  the 
dynamo  and  accumulators  pass  from  the  private  laboratory  to 
the  electrolytic  tables  in  the  laboratory  of  instruction.  An 
amperemeter  shows  the  current  which  is  there  being  used, 
while  another  amperemeter  serves  to  control  the  current  used 
for  charging  the  accumulators.  The  complete  arrangement 
of  the  plant  is  explained  by  Table  I,  Fig.  2. 

That  accumulators  furnish  the  most  suitable  source  of 
current  for  electrolysis  is  to-day  beyond  question.  These 
instruments,  since  they  can  also  be  charged  with  a  thermopile, 
are  more  practical  and  convenient  for  small  laboratories  than 
primary  batteries,  which  furnish  either  insufficient  or  incon- 
stant currents. 

The  torsion  galvanometer  has  long  served  in  this  labora- 
tory for  measuring  the  tension  at  the  electrodes.  Since  a 
knowledge  of  the  tension  is  of  great  importance,  both  for  the 


124        QUANTITATIVE   ANALYSIS    BY    ELECTROLYSIS. 

quality  of  the  precipitated  metal  and  for  the  separation  of 
different  metals,  it  must  always  be  possible,  as  has  been 
repeatedly  stated,  to  determine  the  tension  as. well  as  the  cur- 
rent strength.  How  this  may  be  accomplished  with  a  single 
voltmeter,  in  the  performance  of  several  simultaneous  ex- 
periments by  means  of  a  common  source  of  current,  is  clear 
from  the  diagram,  Table  I,  Fig.  2. 

When  this  method  is  used,  special  care  must  be  taken  that 
the  rheostat  is  connected  between  the  cell  and  the  negative 
pole,  since  otherwise  the  tension  of  the  accumulators  and 
not  that  of  the  cell  will  be  measured.  In  carrying  out  the 
measurement,  only  the  cathode  should  be  connected  with  the 
voltmeter  circuit,  if  a  deflection  at  the  voltmeter  is  expected. 
The  manner  of  attaching  the  cell  is  readily  seen  from  the 
sketch.  Since  the  measurement  of  several  tensions  cannot  be 
conducted  at  the  same  time,  owing  to  one  interfering  with  the 
other,  it  must  always  be  ascertained,  before  switching  in  the 
voltmeter,  that  it  is  not  in  use  elsewhere. 


PRESENT   EQUIPMENT   OF  THE   ELECTROCHEMICAL 
INSTITUTE  OF  THE  TECHNICAL  HIGH  SCHOOL. 

Although  the  electric  current  of  the  former  equipment 
was  furnished  by  an  independent  generating  plant,  such  is  not 
the  case  in  the  present  one.  It  was  considered  desirable  to  be 
as  independent  of  a  power  plant  as  possible,  since  these  are 
by  nature  uneconomical,  and  moreover  are  not  always  ready 
for  use. 

The  solution  of  the  problem  was  made  possible  by  the  fact 
that  the  city  of  Aachen  has  an  electric-power  station,  the 
cables  of  which  extend  to  the  Technical  High  School.  It 
was  decided,  therefore,  to  take  the  electricity  for  the  new 
installation  from  the  city  mains. 


THE   ELECTRO-CHEMICAL  INSTITUTE   AT   AACHEN.   125 

The  current,  as  taken  from  the  city  system,  could  not,  of 
course,  be  used  for  all  experiments  without  modification.  As 
is  well  known,  for  carrying  out  many  electro-analyses,  only 
very  low  tensions,  included  within  the  limits  0.5-8  volts,  are 
required. 

The  current  furnished  by  the  Aachen  Electrical  Works, 
operating  on  the  three- wire  direct-current  system,  has  a  ten- 
sion of  about  108  volts  between  the  middle  wire  and  an  out- 
side wire,  and  a  tension  of  about  216  volts  between  the  two 
outside  wires. 

It  was  therefore  necessary,  in  connection  with  the  experi- 
ments previously  mentioned,  to  reduce  the  high  tension  of 
the  power  wire  in  some  suitable  manner  to  the  low  tension 
required  for  experiment.  Moreover  it  should  at  all  times  be 
possible,  without  special  preparation,  to  carry  out  experiments 
with  high  tension,  as  for  example  in  experiments  where  the 
current  must  be  forced  through  materials  having  a  high 
resistance,  or  for  performing  experiments  with  the  electric  arc. 

For  the  reduction  of  the  high  tension  to  a  low  tension  in 
the  case  at  hand,  a  direct- cur  rent  transformer  was  considered, 
The  economical  working  of  a  double- dynamo  combination  of 
this  description,  its  quiet  and  convenient  operation,  together 
with  the  small  space  which  it  occupies,  all  speak  for  the 
choice  of  the  direct-current  transformer.  The  question  as 
to  the  method  of  obtaining  the  low  tension  required  for 
electrolytic  experiments  was  thus  solved. 

Before  proceeding  to  the  description  of  the  plant  installed 
by  the  firm  of  Schuckert  &  Co.,  proprietors  of  the  Aachen 
Electrical  Works,  the  nature  of  the  different  experiments  and 
investigations  carried  out  will  be  briefly  sketched  in  order 
that  what  follows  may  be  more  readily  comprehended. 


126        QUANTITATIVE  ANALYSIS   BY   ELECTROLYSIS. 

1.  EXPERIMENTS  WITH  Low  TENSION. 

The  experiments  with  low  tension  are  chiefly  confined  to 
the  electro-analysis  of  solutions  of  metallic  salts.  In  addition 
to  this  the  precipitation  of  metals  on  a  large  scale  is  also 
undertaken  as  an  introduction  to  the  study  of  electroplating. 

2.  EXPERIMENTS  WITH  HIGH  TENSION. 

The  experiments  with  high  tension  begin  at  about  45 
volts,  required  for  the  production  of  the  Davy  arc ;  the  high- 
est available  tension  of  the  supply  circuit,  that  between  the 
two  outside  wires,  is  about  216  volts.  The  high-tension 
current  is  chiefly  employed  for  fusion  experiments,  as  well 
as  for  the  decomposition  of  gases  and  other  bodies  of  high 
resistance. 

In  addition  to  the  above  purposes,  the  current  is  also 
used  for  running  an  electric  projection  lantern,  as  well  as  a 
number  of  arc  and  incandescent  lights. 

The  distribution  of  the  current  to  the  various  rooms,  and 
also  the  operation  of  the  transformer,  is  controlled  from  a 
central  switchboard.  The  centralization  of  the  whole  plant 
was  desirable  for  many  reasons,  chief  among  which  was  the 
fact  that  a  valuable  and  complicated  switchboard  might  thus 
be  placed  under  competent  supervision  in  a  room  not  open  to 
every  one.  This  would  prevent  unauthorized  persons  from 
taking  off  current,  and  besides  would  allow  a  general  super- 
vision of  the  whole  plant. 

From  the  central  switchboard  currents  are  carried  to  the 
following  places: 

1.  Private  laboratory. 

2.  Large  lecture-room. 

3.  Laboratory  for  electro-analysis. 


THE  ELECTRO-CHEMICAL   INSTITUTE  AT  AACHEN.    127 

4.   Laboratory  for  experiments  on  a  large  scale  with  high 
and  low  tension. 

The  circuits  running  to  the  different  rooms  are  distin- 
guished, according  to  the  purpose  for  which  the  current  they 
carry  is  intended,  as : 

a.  Lighting  circuits, 
1).  High-tension  circuits, 
c.   Low-tension  circuits, 
and  are  entirely  independent  of  one  another. 

The  lighting  circuits  run  to  the  private  laboratory  and  to 
the  large  lecture-room. 

The  circuits  for  the  high-tension  current  extend  to  the 
private  laboratory,  to  the  large  lecture-room,  and  to  the 
laboratory  for  experiments  with  high-  and  low-tension. 

The  circuits  for  low-tension  current  extend  to.  the  private 
laboratory,  the  large  lecture-room,  the  laboratory  for  electro- 
analysis,  and  the  research  room  for  high  and  low  tension, 

In  addition  to  the  above  circuits,  which  are  intended  for 
direct  current  transmission,  there  are  also  to  be  mentioned 
the  circuit  for  charging  the  two  batteries  of  accumulators  and 
the  circuit  for  running  the  transformer. 

The  switches,  resistances,  controlling,  and  measuring 
apparatus  belonging  to  the  different  circuits  are  located  on 
the  central  switchboard. 

1.  PRIVATE  LABORATORY. 

Concerning  the  special  arrangements,  the  private  labora- 
tory will  next  be  mentioned.  As  already  stated,  the  central 
switchboard,  with  the  apparatus  for  the  control  of  the  whole 
plant,  is  placed  in  the  private  laboratory.  In  tins  room  there 
is  also  located  a  battery  of  accumulators. 

Table  III  gives  a  photographic  view  showing  the  arrange- 
ment of  this  laboratory.  In  the  middle  may  be  seen  the 


128         QUANTITATIVE  ANALYSIS   BY   ELECTROLYSIS. 

central  switchboard  upon  which  the  various  instruments  are 
mounted;  to  the  left  stands  the  glass  closet  containing  the 
battery  of  accumulators.  On  the  wall  by  the  window  are  two 
work-benches,  one  intended  especially  for  electro-analytical 
work  with  low  tensions  and  small  currents,  and  the  other  for 
experiments  with  high  tensions  and  large  currents. 

The  arrangement  for  electro-analysis  is  as  follows:  On 
the  back  of  the  corner  bench  is  a  slanting  wooden  frame,  on 
the  face  of  which  are  fastened  the  switches  and  branch  bind- 
ing-posts, while  the  connecting  wires  are  attached  to  the  back« 
There  are  altogether  five  work-places  on  this  bench,  each  of 
which  will  permit  of  the  performance  of  two  analyses  simul- 
taneously, so  that  in  all  ten  experiments  may  be  carried  on 
at  the  same  time. 

The  installation  of  these  work-places,  as  well  as  of  the 
second  work-bench,  is  in  accordance  with  the  scheme  for 
current  distribution  shown  in  Table  IY. 

Each  work-place  is  connected  in  parallel  to  the  positive 
and  negative  conductors,  which  are  run  through  the  work- 
bench. 

The  current  for  every  analysis  can  be  independently  varied 
by  means  of  the  regulating  resistance  at  the  work-place.  A 
single  amperemeter,  which  can  be  thrown  into  the  circuit  of 
any  analysis  by  means  of  a  switch  placed  at  each  work-place, 
serves  for  measuring  the  current  strength.  When  the  am- 
peremeter is  cut  out,  its  place  is  taken  by  a  resistance,  in  or- 
der that  the  current  strength  may  not  be  altered  (see  p.  122). 

The  measurement  of  the  tension  is  carried  out  in  a  similar 
manner  by  a  single  voltmeter,  which  may  at  will  be  switched 
into  the  circuit  of  any  analysis  in  operation. 

A  lead  safety  fuse  is  inserted  in  the  circuit  of  each  of  the 
ten  branches,  to  guard  against  the  possibility  of  too  great  cur- 
rent strength. 


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THE    ELECTRO-CHEMICAL   INSTITUTE   AT   AACHEN.    129 

The  connections  of  the  electrolytic  apparatus  to  the  small 
switchboards  of  the  work-bench  are  made  with  very  flexible 
rubber-insulated  copper  leads,  the  ends  of  which  are  provided 
with  small  copper  links  to  allow  of  a  more  convenient  attach- 
ment to  the  apparatus. 

For  setting  up  experiments  where  large  currents  of  high 
or  low  tension  are  required,  two  cases  furnished  with  locks 
are  affixed  to  the  second  work-bench.  That  for  low  tension, 
contains  two  branch  plates  which  carry  a  number  of  binding- 
posts,  thus  allowing  several  different  pieces  of  apparatus  to  be 
connected  at  the  same  time. 

The  case  for  high  tension  contains  three  plates,  connected 
with  the  two  outside  leads  and  the  middle  lead  of  the  three- 
wire  system  respectively,  whereby  a  maximum  tension  of 
about  216  volts  is  obtainable.  These  plates  also  carry  several 
binding-posts,  which  permit  the  use  of  several  pieces  of  appa- 
ratus at  one  time. 

The  two  accumulator  batteries  are  comprised  of  four  cells 
each.  One  battery,  with  the  cells  connected  in  series,  requires 
a  charging  current  of  90  amperes ;  the  other,  similarly  con- 
nected, requires  25  amperes. 

The  batteries  are  charged  from  the  transformer. 

The  small  battery  furnishes  current  to  the  private  labora- 
tory only,  while  the  large  one  supplies  the  rest  of  the  plant. 
Each  of  the  batteries  is  provided  with  a  cell  switchboard  for 
four  cells,  so  that  by  cutting  out  separate  cells  the  tension  of 
the  current  may  be  reduced  and  the  use  of  higli  external  re- 
sistances avoided. 

As  a  protection  against  the  possibility  of  the  current  re- 
versing, during  the  process  of  charging,  and  flowing  back 
through  the  transformer,  each  battery  circuit  is  provided  with 
an  automatic  cut-out. 

The  tension  of  the  separate  cells  is  controlled  by  a  special 


130        QUANTITATIVE  ANALYSIS   BY   ELECTROLYSIS. 

voltmeter  having  contact  plugs,  which  allows  the  tension  of 
each  cell  to  be  independently  measured  at  the  cell  switch- 
board. 

For  the  measurement  of  the  battery  tension  and  the 
strength  of  the  charging  and  discharging  currents  a  special 
voltmeter  and  amperemeter  are  provided.  Further,  that  the 
operation  of  charging  and  discharging  may  be  more  closely 
observed,  indicators  for  showing  the  direction  of  the  current 
are  attached  to  the  corresponding  circuits. 

2.   LARGE  LECTURE-ROOM. 

The  installation  of  the  large  lecture-room  is  especially  in- 
tended for  the  performance  of  lecture  experiments,  which 
comprise  the  demonstration  of  electrolysis,  the  decomposition 
of  gases  and  liquids  by  the  Davy  arc,  and  fusion  experiments. 

Besides  this,  provision  is  made  for  the  running  of  an 
electric  projecting  lantern,  as  well  as  for  a  number  of  incan- 
descent and  arc  lamps. 

3.   LABORATORY  FOR  THE  ELECTRO- ANALYSIS  OF  METALS. 

In  this  room  the  transformer  is  placed.  It  also  contains 
a  large  experiment  table  having  ten  work -places,  for  carrying 
out  electro-analytical  experiments  with  low  tensions.  (Gf. 
Table  Y.) 

The  transformer  will  next  be  described.  This  consists 
of  a  combination  of  two  direct-current  dynamos,  with  their 
shafts  coupled  directly  together.  One  of  the  dynamos,  ar- 
ranged as  a  motor,  is  driven  by  the  current  from  the  two 
outside  wires  of  the  three-wire  system,  by  a  tension,  there- 
fore, of  about  216  volts.  The  circuit  is  run  to  the  trans- 
former from  the  central  switchboard.  The  dynamo  which  is 
coupled  to  the  motor,  and  which  furnishes  the  low-tension 


THE  ELECTRO-CHEMICAL   INSTITUTE  AT   AACHEN.   131 

current,  is  so  arranged  that  the  tension  at  the  poles  may  be 
varied  from  about  4.5-9  volts,  the  corresponding  current 
strengths  being  respectively  360  and  180  amperes.  The  con- 
ductors carrying  the  low-tension  current  from  the  dynamo 
run  to  the  central  switchboard.  The  tension  of  9  volts  is 
the  one  generally  used,  the  lower  tension  "of  4.5  volts  being 
employed  for  larger  electrolytic  experiments,  such  as  the  prep- 
aration of  pure  metals. 

The  alteration  in  the  tension  of  the  current  is  brought 
about  by  connecting  the  two  halves  of  the  double  armature, 
with  which  the  transformer  is  provided,  either  in  series  or  in 
parallel.  This  is  done  by  merely  changing  the  corresponding 
connections  on  the  frame  of  the  transformer. 

Further  concerning  the  construction  of  the  transformer,  it 
should  be  mentioned  that  the  machine  is  very  solidly  cast,  and 
the  magnets  protected  within  the  frame,  so  that  a  mechanical 
injury  to  the  magnet-coils  is  out  of  the  question.  The  lubri- 
cation of  all  parts  is  carried  out  by  means  of  ring-lubrication, 
which  has  proved  very  satisfactory.  Such  delays  as  often 
occur  when  other  mechanical  contrivances  are  employed  are 
here  impossible.  Owing  to  its  construction,  the  transformer, 
which  for  protection  is  enclosed  in  a  special  covering,  can  run 
for  hours  without  particular  attention. 

The  action  of  the  transformer,  in  spite  of  its  speed  of  about 
1300  revolutions  per  minute,  is  so  quiet  and  free  from  any 
jarring  or  shaking,  that  its  running  can  scarcely  be  detected 
even  in- the  immediate  neighborhood. 

It  should  be  stated  that  there  is  a  switchboard  near  the 
transformer,  by  which  direct  currents  of  low  tension  can  be 
taken  off  in  this  room,  without  making  use  of  the  central 
switchboard.  Such  currents  are  required  when  experiments 
with  high  current  strength  and  low  tension  are  performed ; 


132        QUANTITATIVE  ANALYSIS   BY   ELECTROLYSIS. 

and  in  such  cases  short  cables  are  run  from  this  switchboard 
to  the  nearest  work-bench,  where  the  apparatus  is  set  up. 

The  arrangement  of  the  large  work-bench,  a  photograph 
of  which  is  given  in  Table  Y,  corresponds  in  general  to  that 
of  the  table  for  conducting  analyses  in  the  private  labora- 
tory. 

Here,  on  either  side  of  the  bench,  there  are  five  work- 
places, each  of  which  allows  of  the  simultaneous  performance 
of  two  analyses,  so  that  in  all  twenty  experiments  may  be 
carried  on  at  the  same  time.* 

Table  IY  shows  the  method  employed  for  measuring  the 
current  strength  and  tension  of  an  analysis.  The  ampere- 
meter and  voltmeter  are  above.  The  current  strengths  are 
regulated  by  means  of  the  rheostats  (I,  II,  III,  and  IY). 
These  consist  of  slate  slabs  into  which  are  fixed  metal  knobs, 
which  are  attached  to  separate  resistance  spirals.  By  turning 
the  lever  in  the  direction  indicated  by  the  arrow,  the  resist- 
ance is  cut  out  and  the  current  strength  correspondingly 
increased. 

The  switches  for  the  amperemeter  A(I(  n>  m  IV) ,  f or  the 
electrolyses  i  E(I|  IIt  IIIt  IV) ,  and  the  safety-fuses  B(Ii  n>  IIIf  IV)  are 
located  under  bronzed  metal  cases. 

%,  ii,  in,  iv)  are  the  binding-posts  to  which  the  electrolyses 
are  connected,  Y  is  a  double-pole  switch  used  in  measuring 
the  tension.  In  the  position  o  the  voltmeter  is  cut  out ;  at 
i,  n,  in,  iv  the  corresponding  electrolysis  is  connected  with 
the  voltmeter.  As  already  stated,  there  is  only  one  ampere- 
meter and  one  vo.ltmeter  to  every  table  with  10-20  dishes, 
and  therefore  only  one  electrolysis  can  be  measured  at  a  time. 
The  four  figures  on  Table  IY  are  designed  to  make  the 
explanations  clearer. 

*  Two  other  work-benches  have  been  recently  added,  so  that  there  are 
now  twenty  work-places  for  electro-analysis. 


THE    ELECTRO-CHEMICAL   INSTITUTE   AT  AACHEN.    133 

In  position  I,  where  the  keys  Ar  and  Er  are  horizontal, 
the  circuit  is  closed.  In  II,  An  is  perpendicular;  the 
amperemeter  is  in  circuit.  The  lever  of  the  rheostat  at  II 
may  be  turned  in  the  direction  of  the  arrow  until  the  meas- 
uring apparatus  registers  the  desired  current  strength.  A  is 
then  brought  into  the  position  Am  and  E  to  Em.  The  cur- 
rent now  flows  no  longer  through  the  amperemeter,  but 
through  a  roll  of  wire,  the  resistance  of  which  is  equal  to 
that  of  the  amperemeter.  The  current  strength  remains  the 
same  as  that  previously  shown  by  the  amperemeter. 

Y  serves  for  measuring  the  tension  at  the  poles  of  the 
electrolytic  vessel,  as  shown  at  YIV.  In  this  operation  the 
position  of  A  and  E  is  the  same  as  in  III.  The  two  metal 
strips  (SS)  are  pushed  to  the  right  or  left  (in  the  figure  to 
the  right,  iv),  whereupon  the  voltmeter  shows  the  tension 
existing  at  that  time  at  the  poles  of  the  corresponding  elec- 
trolysis. The  instruments  should  be  cut  out  immediately 
after  use. 

4t.  LABORATORY  FOR  PERFORMING  EXPERIMENTS  ON  A  LARGE 
SCALE  WITH  Low  AND  HIGH  TENSIONS. 

As  already  mentioned,  special  cases  wrhich  receive  their 
currents  from  separate  conductors  running  from  the  central 
switchboard  are  arranged  for  high  and  low  tension. 

Within  the  case  for  high  tension  there  are  three  separate 
plates  corresponding  to  the  three  wires  of  the  three- wire 
system,  providing  currents  at  tensions  of  108  and  216  volts 
accordingly. 

The  case  for  low  tension  contains  two  connections,  with 
possible  tension  at  the  poles  up  to  9  volts. 

From  both  of  the  cases  separate  branch  circuits  run  to  the 
four  work-benches,  where  they  end  in  terminal  boxes  pro- 


134         QUANTITATIVE  ANALYSIS   BY   ELECTEOLYSIS. 

vided  with  locks.     By  this  arrangement  each  table  is  pro- 
vided with  both  high  and  low  tension. 

Each  of  the  branches  running  to  the  tables  is  supplied 
with  a  safety  fuse  and  a  switch;  each  table  is  therefore 
independent  of  the  others. 

A  set  of  transportable  resistances  and  measuring  instru- 
ments for  regulating  the  current  is  used  in  carrying  out 
experiments. 

Large  and  cumbersome  resistances  are  required  to  produce 
appreciable  variations  in  the  tension.  A  simple  appliance  in 
use  in  the  Aachen  laboratory  overcomes  this  difficulty  in  the 
case  of  experiments  of  short  duration,  where  economical  use 
of  the  current  is  not  an  essential  feature.  This  scheme, 
originated  by  Lob  and  Kaufmann,*  permits  the  convenient 
splitting  up  of  the  current  of  216  or  108  volts  into  separate 
independent  currents  having  the  required  lower  tension. 

A  number  of  lead  plates  are  hung  parallel  to  one  another 
in  a  large  porcelain  trough  filled  with  sulphuric  acid  (Fig.  84), 

in  such  a  manner  that  they  cut 
all  the  lines  of  the  current. 
They  must  therefore  almost 
touch  the  sides  and  bottom  of 
the  trough.  When  the  current 
passes,  these  lead  plates  act  as 
intermediate  conductors,  the  sum 
of  their  separate  tensions  being 
equal  to  the  tension  of  the  main 
current.  The  arrangement  is  of  course  impractical  as  an 
accumulator,  since  the  polarized  plates  immediately  short- 
circuit  through  the  electrolyte  and  are  reduced  to  the  poten- 
tial of  the  electrodes. 


FIG.  84. 


*  Zeitschr.  f.  Elektroch.,  1895-96,  p.  345.     Ibid.,  p.  664. 


THE   ELECTRO-CHEMICAL    INSTITUTE   AT   AACHEN.  135 

The  immersed  lead  plates  can  be  slid  along  the  length  of 
the  trough  on  the  glass  rod  by  which  they  are  hung.  By 
moving  the  plates  toward  or  away  from  the  electrodes  the 
tension  is  varied,  and  any  desired  tension  may  be  obtained  by 
making  a  connection  between  a  terminal  electrode  and  one  of 
the  plates.  The  arrangement  is  given  in  Fig.  84.  E  de- 
notes the  source  of  current ;  T,  the  trough  filled  with  sul- 
phuric acid ;  A  and  K,  anode  and  cathode ;  M,  the  five  plates. 
The  wires  to  S  show  the  removal  of  three  separate  currents 
of  different  tensions.  A  large  number  of  such  connections 
are  possible.  On  account  of  the  gases  given  off,  the  trough 
should  be  kept  under  a  hood. 

In  addition  to  the  details  of  the  equipment  which  have 
been  described,  some  general  facts  in  connection  with  the 
management  of  the  entire  plant  should  be  stated. 

Since  the  apparatus  is  much  used,  and  is  not  always 
placed  in  experienced  hands,  it  was  considered  desirable  to 
have  all  parts  solidly  constructed  and  intended  for  continu- 
ous use. 

The  switches  and  regulating  instruments,  as  well  as  the 
branch  plates,  are  all  mounted  on  bases  of  fire-proof  material. 

All  connections  are  made  with  the  best  rubber-covered 
wire,  fastened  to  large  porcelain  brackets,  so  that  most  perfect 
insulation  of  the  conductors  is  assured. 

To  secure  against  improper  use,  all  switch-cases  are  pro- 
vided with  safety-locks,  so  that  currents  can  nowhere  be 
taken  off  without  the  permission  of  the  director  of  the 
laboratory. 


SECTION  II. 
SPECIAL  PART. 


QUANTITATIVE     DETERMINATION     OF     THE 
METALS.  * 

IRON. 

LITEEATUBE  I 

Wrightson,  Zeit.  f.  analyt.  Chem.,  15,  305. 

Luckow,  Zeit.  f.  analyt.  Chem.,  19,  18. 

Classen  and  v.  Keiss,  Ber.  deutsch.  chem.  Ges.,  14,  1622. 

Classen,  Zeit.  f.  Elektrochemie,  vol.  I. 

Moore,  Chem.  News,  53,  209. 

Smith,  Amer.  Chem.  Jour.,  10,  330. 

Brand,  Zeib.  f.  analyt.  Chem.,  28,  581. 

Drown  and  Meckenna,  J.  of  Analyt.  and  Applied  Chem.,  5,  627. 

Smith  and  Muhr,  ibid.,  5,  488. 

Rudorff,  Zeit.  f .  angew.  Chem.,  15,  198. 

Vortmann,  Monatshefte  f.  Chem.,  14,  542. 

Heidenreich,  Ber.  deutsch.  chem.  Ges.,  29,  1585. 

If  the  solution  of  a  ferrous  salt  f  is  treated  with  potassium 

*  Of  the  methods  existing  in  the  literature,  reference  will  be  made  only 
to  those  which  give  the  necessar}r  and  complete  details  concerning  the 
conditions  of  experiment. 

f  As  stated  on  p.  5,  sulphates  are  best  adapted  to  this  treatment,  chlo- 
rides less  so,  while  nitrates  must  be  avoided.  The  presence  of  phosphoric 
acid  is  not  harmful. 

137 


138         QUANTITATIVE  ANALYSIS   BY    ELECTROLYSIS. 

or  ammonium  oxalate,  there  is  produced  an  intensely  yellow- 
ish-red precipitate  of  ferrous  oxalate,  soluble  in  an  excess  of 
the  reagent  to  a  yellowish-red  solution  of  the  double  salt. 

The  above-named  oxalates  do  not  precipitate  ferric  salts ; 
but,  if  added  in  sufficient  quantity,  a  solution  of  the  double 
ferric  salt  is  produced  having  a  more  or  less  green  color.  If 
this  solution  is  submitted  to  electrolysis,  there  is  first  produced 
the  double  ferrous,  salt,  which  is  then  decomposed  with  separa- 
tion of  metallic  iron ;  the  green  liquid  therefore  becomes  first 
red,  and  then  colorless.  Because  of  this  action,  the  determina- 
tion of  iron  is  more  rapidly  performed  in  solutions  of  ferrous 
than  of  ferric  salts.  Potassium  iron  oxalate  is  not  adapted  to 
electrolysis,  because  the  potassium  carbonate  which  is  pro- 
duced precipitates  iron  carbonate,  and  thus  complete  reduc- 
tion is  prevented.  The  electrolysis  of  the  ammonium  double 
salt,  when  ammonium  oxalate  is  in  sufficient  excess,  proceeds 
smoothly,  with  no  separation  of  an  iron  compound.  If  the 
solution  contains  free  hydrochloric  acid,  it  is  best  to  remove  it 
by  evaporation  on  the  water- bath. 

Free  sulphuric  acid  may  be  neutralized  with  ammonia, 
since  the  ammonium  sulphate  thus  produced  only  increases 
the  conductivity  of  the  solution.  Nitrates  are  converted  by 
evaporation  with  sulphuric  acid  into  sulphates,  or  by  repeated 
evaporation  with  hydrochloric  acid  into  chlorides. 

The  determination  is  conducted  as  follows:  Assuming 
that  1  g  of  iron  may  be  present  in  the  solution  to  be  elec- 
trolyzed,  6—8  g  of  ammonium  oxalate  are  dissolved  by  heat  in 
as  little  water  as  possible,  and  the  iron  solution  is  gradually 
added,  with  constant  agitation.*  The  solution  is  then  diluted 

*  It  is  not  desirable  to  add  ammonium  oxalate  solution  to  a  ferrous 
solution,  as  difficultly  soluble  ferrous  oxalate  separates,  and  can  be  dis 
solved  to  the  double  salt  only  by  long  heating.  With  a  ferric  solution  this 
precaution  is  unnecessary. 


IRON.  139 

with  water  to  100-150  cc,  and  the  positive  electrode  is  im- 
mersed in  the  liquid  until  it  is  just  covered  by  the  solution. 
The  electrolysis  is  conducted  according  to  the  special  direc- 
tions which  are  given  below. 

The  end  of  the  reaction  is  determined  by  taking  out  a 
small  portion  of  the  colorless  solution  with  a  capillary  tube, 
acidifying  strongly  with  hydrochloric  acid,  and  testing  with 
potassium  sulphocyanate.  When  the  reaction  is  ended  the 
positive  electrode  is  removed  from  the  solution,  which  is 
poured  off,  and  the  dish  washed  three  times  with  cold  water 
(about  5  cc  each  time),  and  three  times  with  absolute  alcohol, 
dried  a  few  moments  in  the  air-bath  at  a  temperature  of  70°  to 
90°,  and  weighed  after  cooling. 

The  separated  iron  has  a  steel-gray  color  and  brilliant 
lustre,  is  firmly  attached  to  the  dish,  and  can  be  preserved  in 
the  air  without  oxidation  for  a  full  day. 


CONDITIONS    OF    EXPERIMENT.* 

Temperature  of  the  liquid:  Although  the  maintenance 
of  a  certain  uniform  temperature  is  not  essential  to  the  suc- 
cess of  the  experiment,  it  has  been  found  in  practice  that 
the  ordinary  temperature  of  the  solution  (20-40°)  is  the 
most  favorable  to  the  rapid  completion  of  the  analysis. 

Current  density,  ND100 :  For  solutions  at  ordinary  tem- 
perature, 1-1.5  amp.;  for  warm  solutions  (40-65°),  0.5-1 
amp. 

Electrode  tension :  For  warm  solutions,  with  the  stated 
current  density,  2.0-3.5  volts;  otherwise,  3.6-4.3  volts. 

*  Method  of  the  author.  In  all  of  the  author's  methods  the  statements 
refer  to  the  use  of  the  electrodes  described  on  p.  85;  the  current  densities 
refer  only  to  the  dish  given  in  Fig.  54. 


140         QUANTITATIVE   ANALYSIS   BY    ELECTROLYSIS. 

For  the  quality  of   the  precipitated   metal,  polished   or 
roughened  dishes  answer  equally  well. 

EXPERIMENT. 

Used  2.1-2.5  g  FeSO4(NH4)2SO4.6H2O,  6-8  g  ammonium 
oxalate,  120  cc  of  liquid. 


Current  Density, 
Amperes. 

1    -1.5 

Electrode  Tension, 
Volts. 

3.85-4.3 

Temp. 
20-40° 

Time. 
2  hr.  15  m. 

Found.* 
14.21  % 

1    -1.05 

3.6  -4.2 

36° 

3  "    50  " 

14.21  " 

1    -1.08 

3.05-3.52 

65° 

2  "    30  " 

14.28  " 

0.5-0.55 

2.0  -2.3 

50-52° 

3  "    30  " 

14.24  " 

Used  2.6-2.8  g  ferric  potassium  oxalate  (Fea(C2O4),. 
3K2C2O4.6H2O),  6-7  g  ammonium  oxalate. 

1.5-1.7  3.55-4.25  35-40°  2  hr.  54  m.        11.39  %'\ 

1.0-1.1  3.9  -4.0  30-40°  3  "    15  "         11.35  " 

0.5-0.8  2.4  -2.8  50°  6  "    15  "         11.25  " 

Edgar  F.  Smith  precipitates  iron  from  a  solution  of  am- 
monium citrate  to  which  a  few  drops  of  citric  acid  have  been 
added.  The  author's  experiments  in  earlier  years  on  the 
separation  of  iron  from  other  metals  in  citric  and  tartaric 
acid  solution,  demonstrated  that  in  the  presence  of  fixed 
organic  acids  the  precipitated  metal  always  contains  carbon. 
Heidenreich  has  recently  shown,  by  experiments  conducted 
in  the  Aachen  laboratory,  that  iron  may  be  quantitatively 
determined  under  certain  conditions,  namely:  0.2  g  ferrous 
ammonium  sulphate,  50  cc  of  a  10  per  cent  solution  of 
sodium  citrate,  2  cc  of  a  saturated  solution  of  citric  acid; 
entire  volume  of  liquid,  120  cc;  temperature  of  room; 
ND100  =  0.75-0.9  amp.;  electrode  tension,  5  volts;  time, 
4r-6  hours.  The  iron,  however,  always  contains  carbon. 


[Theory  14.29*.]  t  [Theory  11.40*.] 


COBALT.  141 


COBALT. 

LITEKATUKE  I 

Gibbs,  Zeit.  f.  anal.  Chem.,  3,  336  ;  11,  10 ;  22,  548. 

Merrick,  Amer.  Chemist,  2,  136. 

Wrightson,  Zeit.  f.  anal.  Chem.,  15,  300,  303.  333. 

Schweder,  ibid.,  16,  344. 

Cheney  and  Richards,  Amer.  Jour,  of  Science  and  Arts,  [3]  14,  178. 

Ohl,  Zeit.  f.  anal.  Chem.,  18,  523. 

Luckow,  ibid.,  19,  314. 

Riche,  ibid.,  21,  116. 

Classen  and  v.  Reiss,  Ber.  deutsch.  chem.  Ges.,  14,  1622.  2771. 

Classen,  ibid.,  27,  2061  ;  Zeit.  f.  Elektrochemie,  1894-95,  Heft  1. 

Schucht,  Zeit.  f.  anal.  Chem.,  21,  493. 

Eohn  and  Woodgate,  Journ.  Soc.  Chem.  Indust.,  8,  256. 

Riidorff,  Zeit.  f.  angew.  Chemie,  1892,  p.  6. 

Brand,  Zeit.  f.  anal.  Chemie,  28,  588. 

Le  Roy,  Compt.  rend.,  112,  722. 

Vortmann,  Monatsh.  f.  Chem.,  14,  536. 

Oettel,  Zeit.  f.  Elektrochemie,  1894-95,  p.  195. 

Fresenius  and  Bergmann,  Zeit.  f.  anal.  Chem.,  19,  329. 

Cobalt  may  be  very  easily  precipitated  from  a  solution  of 
cobalt  ammonium  oxalate  containing  an  excess  of  ammonium 
oxalate  (method  of  the  author).  The  metal  separates  rapidly 
at  the  negative  electrode,  in  a  compact  adherent  coating, 
showing  its  characteristic  metallic  properties.  The  operation 
is  performed  as  in  the  determination  of  iron.  4-5  g  am- 
monium oxalate  are  dissolved  by  heating  in  the  solution,  the 
volume  of  which  should  be  about  25  cc ;  it  is  then  diluted  to 
100-120  cc,  warmed,  and  electrolyzed  at  60-70°. 

CONDITIONS    OF    EXPERIMENT. 

Temperature  of  the  liquid  :  The  period  of  electrolysis  is 
considerably  shortened  by  warming,  so  that  a  temperature  of 
60-70°  is  suitable. 


142        QUANTITATIVE  ANALYSIS   BY   ELECTROLYSIS. 

Current  density  :  The  proper  current  density  for  warmed 
solutions  is  KD100  =  1  amp. 

Electrode  tension  :    3.1-3.  8  volts. 

The  condition  of  the  surface  of  the  cathode  has  no  effect 
upon  the  quality  of  the  precipitated  metal. 

EXPERIMENT. 

Used  2.2-2.6  g  CoSO4.K1SO4.6H,O,  4-5  g  ammonium 
oxalate,  120  cc  solution. 


A.mperes. 

jtLiieciroue  lensi 
Volts. 

Temp. 

Time. 

Found.* 

1     -1.1 

3.1  -3.78 

60-65° 

2  br.   15  ra. 

13.36  £ 

0.5-0.52 

2.7  -2.95 

60-65° 

3  "     30  " 

13.49" 

1    -1.2 

3.9  -4.1 

15-35° 

4  "     30  •' 

13.43" 

0.5-0.53 

3.46-3.9 

15-27° 

6  "    35  " 

1325" 

According  to  a  method  given  by  Fresenius  and  Bergmann, 
the  cobalt  solution,  after  the  addition  of  15-20  cc  of  an 
ammonium  sulphate  solution  (300  g  (NH4),SO4  to  the  liter) 
and  40  cc  ammonia  sp.  g.  0.96  (where  more  than  0.5  g 
cobalt  is  present  in  the  solution,  50-60  cc  NH4OH),  is 
diluted  with  water  to  150-170  cc,  and  electrolyzed  with  a 
current  of  I^D100  =  0.7  as  a  maximum  at  ordinary  tempera- 
tures. The  presence  of  chlorides  and  nitrates  is  unfavorable 
to  the  reduction.  Fixed  organic  acids  (citric  acid,  tartaric 
acid)  and  also  magnesium  compounds  act  injuriously. 

CONDITIONS    OF    EXPERIMENT. 

Temperature  of  the  liquid  :  The  separation  is  not  hastened 
by  warming. 

Current  density:   JSTDIOO  =  0.5-0.7  amp. 

Electrode  tension  :  With  the  given  current  density  and  at 
ordinary  temperatures,  this  equals  2.8-3.3  volts. 

F.  Oettel  proposes  the  following  method  for  the  determina- 
tion of  cobalt  :  The  salt  is  dissolved  in  water  and  a  quantity 
*  [Theory  13.43^.] 


NICKEL.  143 

of  ammonium  chloride,  equal  to  four  times  the  weight  of  the 
salt  taken,  is  added.  The  volume  of  the  liquid  is  150  cc, 
-J-  of  which  is  an  ammonia  solution  (sp.  g.  =  0.92).  After 
electrolyzing  for  14  hours  the  cobalt  is  quantitatively  pre- 
cipitated if  100  cc  of  solution  do  not  contain  more  than  0.25 
g  of  the  cobalt  salt. 

NICKEL. 

LITERATURE  I 

Gibbs,  Zeit.  f.  anal.  Chem.,  3,  336 ;  11,  10  ;  22,  558. 

Merrick,  Amer.  Chemist,  2,  136. 

Wrightson,  Zeit.  f.  anal.  Chem.,  15,  300,  303,  333. 

Schweder,  ibid.,  16,  344. 

Cheney  and  Richards,  Amer.  Journ.  of  Science  and  Arts,  [3]  14,  178. 

Ohl,  Zeit.  f.  anal.  Chem.,  18,  523. 

Luckow,  ibid.,  19,  314. 

Riche,  ibid.,  21,  116. 

Classen  and  v.  Keiss,  Ber.  deutsch.  chem.  Ges.,  14,  1622,  2771. 

Classen,  ibid.,  27,  2061 ;  Zeit.  f.  Elektrochemie,  1894-95,  Heft  1. 

Schucht,  Zeit.  f.  anal.  Chem.,  21,  493. 

Kohn  and  Woodgate,  Journ.  Soc.  Chem.  Indust.,  8,  256. 

Riidorff,  Zeit.  f.  angew.  Chem.,  1892,  p.  6. 

Brand,  Zeit.  f.  anal.  Chem.,  28,  588. 

Le  Roy,  Compt.  rend.,  112,  722. 

Vortmann,  Monatsh.  f.  Chemie,  14,  536. 

Campbell  and  Andrews,  Journ.  Am.  Chem.  Soc.,  17,  125. 

Oettel,  Zeit.  f.  Elektrochemie,  1894-95,  p.  192. 

Fresenius  and  Bergmann,  Zeit.  f.  anal.  Chem.,  19,  320. 

Nickel  may  be  reduced  under  conditions  similar  to  those 
requisite  for  cobalt ;  the  metal  is  precipitated  from  the  solu- 
tion of  the  double  oxalate  containing  ammonium  oxalate  in 
excess,  by  the  action  of  a  similar  current,  as  a  thick,  bright 
coating  on  the  negative  electrode.  The  end  of  the  reaction 
is  ascertained  by  testing  with  ammonium  sulphide  or  potas- 
sium sulphocarbonate,  and  the  precipitate  is  treated  as  pre- 
viously directed. 


144         QUANTITATIVE   ANALYSIS  BY    ELECTROLYSIS. 
CONDITIONS    OF    EXPERIMENT . 

The  conditions  of  experiment  are  the  same  as  in  the 
separation  of  cobalt.  Here  also  polished  or  roughened  dishes 
serve  equally  well. 

Used  1.2-2.1  g 'NiSO4.(]NTH4)2SO4.6H2O,  4-5  g  ammo- 
nium oxalate,  120  cc  liquid. 

Current  Density,    Electrode  Tension,         Temp  Time  Found.* 

ArnpGrGS.  vtMio. 

0.9-1  3.1-2.9  65-70°  2  hr.  50  m.            15.13  # 

0.5-0.6  3.38-3.4  17°  5"  15.11" 

0.9-1  4.09-4.35  15-30°  3  "  35  "             15.05" 

05-0.53  2.7-2.85  60-65°  4"  15.17" 

The  condition  of  the  precipitate  is  best  when  the  elec- 
trolysis is  conducted  at  a  temperature  of  60-70°,  with  a 
current  of  ND100  =  1  amp. 

According  to  Fresenius  and  Bergmann,  nickel,  like  cobalt, 
may  be  precipitated  completely  from  a  solution  treated  with 
ammonium  sulphate  and  ammonia  (see  Cobalt). 

The  method  given  by  Oettel  for  the  determination  of 
cobalt  may  also  be  used  for  nickel.  For  this  purpose  the 
nickel  salt  is  dissolved  in  20-40  cc  ammonia  (sp.  g.  —  0.92)  to 
which  10  g  ammonium  chloride  are  added,  and  after  diluting 
to  about  120  cc,  the  nickel  is  precipitated  in  7-8  hours  with 
a  current  of  ND100  =  0.45  amp. 

Campbell  and  Andrews  dissolve  nickel  hydroxide  in  30  cc 
of  a  10  per  cent  solution  of  di-sodium  phosphate,  to  which  30 
cc  of  a  concentrated  ammonia  solution  are  added,  and,  with  a 
distance  of  5  mm  between  the  electrodes,  separate  out  the 
nickel  by  the  use  of  a  current  of  KD100  =  0.14  amp. 

ZINC. 
LITERATURE  I 

Wrightson,  Zeit.  f.  anal.  Chem.,  15,  303. 
Parodi  and  Mascazzini,  Ber.  deutsch.  chem.  Ges.,  10,  1098 ; 
Zeit.  f.  anal.  Chem.,  18,  587. 

*  [Theory  14.87$  Ni.] 


ZINC.  145 

Riche,  Zeit.  f.  anal.  Chem.,  17,  216. 

Beilstem  and  Jaweiu,  Ber.  deutsch.  chem.  Ges.,  12,  446 ; 

Zeit.  f.  anal.  Chein.,  18,  588. 
Riche,  Zeit.  f.  anal.  Chem.,  21,  119. 
Reinhardt  and  Ihle,  Journ.  f.  prakt.  Chem.,  24,  193. 
Classen  and  v.  Reiss,  Ber.  deutsch.  chem.  Ges.,  14,  1622. 
Classen,  ibid.,  27,  2060.       x- 
Gibbs,  Zeit.  f.  anal.  Chem/  22,  558. 
Luckow,  ibid.,  25,  113. 
Brand,  ibid.,  28,  581. 
Warwick,  Zeit,  f.  anorg.  Chem.,  1,  290. 
Vortmann,  Ber.  deutsch.  chem.  Ges.,  24,  2753. 
Riidorff,  Zeit.  f.  angew.  Chem.,  1892,  p.  197. 
Vortmann,  Monatsh.  f.  Chemie,  14,  536. 
Jordis,  Zeit.  f.  Elektrochemie,  1895-96,  pp.  138,  565,  655. 
Millot,  Bull,  de  la  Soc.  chim.,  37,  339. 

The  metal  may  be  easily  and  quickly  separated  from  the 
double  salts  of  zinc  ammonium  oxalate  and  zinc  potassium 
oxalate  (method  of  the  author).* 

The  reduced  metal  has  a  bluish-white  color,  and  under 
proper  conditions  adheres  firmly  to  the  negative  electrode. 
Indeed,  the  metallic  zinc  often  adheres  so  firmly  to  the 
platinum  dish  that,  after  being  cleaned  with  water  and  alco- 
hol, and  dried,  it  is  with  difficulty  dissolved  by  warming 
with  acids.  Generally,  after  this  operation,  a  dark  coating  of 
platinum- black  remains  which  can  only  be  removed  by  ignit- 
ing the  dish  and  again  treating  with  acids.  It  is  therefore 
desirable,  before  weighing  the  dish,  to  precipitate  upon  it  a 
thin  coating  of  copper,  tin,  or,  better,  silver.  In  laboratories 


*  The  reduction  of  zinc  from  a  solution  of  zinc  ammonium  oxalate  is 
very  often  credited  to  Reinhardt  and  Ihle.  The  author,  however,  described 
this  method  in  Fehling's  "  Handworterbuch  "  before  the  research  of  the 
above-named  investigators  appeared  in  the  Journal  f lir  praktische  Chemie, 
to  the  editor  of  which,  Kolbe,  the  author  especially  stated  the  facts  at  the 
time. 


146         QUANTITATIVE  ANALYSIS   BY   ELECTKOLYSIS. 

in  which  many  zinc  determinations  are  performed,  silver 
dishes  may  be  advantageously  employed. 

A  bright,  thick  coating  of  copper  can  be  obtained  in  a  few 
minutes  if  a  saturated  solution  of  copper  sulphate  is  treated 
with  an  excess  of  ammonium  oxalate  to  form  the  double  salt, 
acidified  with  oxalic  acid,  warmed  to  70-80°,  and  decomposed 
by  a  current  of  1  ampere.  The  preparation  of  the  double 
salt  in  a  beaker,  and  the  transfer  of  the  clear,  hot  solution  to 
the  platinum  dish,  is  to  be  recommended. 

For  silvering  the  dish  it  is  best  to  precipitate  the  silver 
from  a  solution  of  the  same  in  potassium  cyanide  (see  Silver). 

In  determining  zinc  by  this  method,  the  zinc  salt  is  dis- 
solved in  a  little  water  by  warming,  about  4  g  of  potassium 
oxalate  or  an  equal  amount  of  ammonium  oxalate  is  added, 
and  the  whole  is  brought  into  solution  by  warming  and,  if 
necessary,  by  the  addition  of  small  quantities  of  water.* 
The  liquid  is  now  transferred  to  a  platinrtm  dish  coated  with 
copper  or  silver,  and  electrolyzed.  The  author  has  demon- 
strated by  experiments  that  the  separation  of  the  zinc  in  a 
dense,  shiny  condition  is  possible  if  the  solution  be  kept  acid 
during  the  process  of  analysis. 

For  acidifying  the  solution,  a  cold  saturated  solution  of 
oxalic  acid,  or,  better,  a  solution,  of  tartaric  acid  (3 : 50)  is  em- 
ployed. At  the  start  the  solution  is  electrolyzed  for  about 
3-5  minutes  without  addition  of  acid,  and  then  the  acid  is 
permitted  to  flow  in  drops  (about  10  drops  per  minute)  from 
a  burette  with  a  fine  outlet,  upon  the  watch-glass  covering 
the  dish.  The  acid  flows  through  the  holes  in  the  watch- 
glass  into  the  dish  itself.  After  the  reduction  is  completed 

*  If  the  alkali  oxalate  be  added  to  a  dilute  solution  of  a  zinc  salt,  there 
first  forms  a  precipitate  of  zinc  oxalate  which  is  not  completely  converted 
into  the  soluble  zinc  double  salt  if  the  solution  of  the  alkali  oxalate  is  too 
dilute. 


ZINC.  147 

(this   is  determined  with  potassium  ferrocyanide),  the  metal 
must  be  washed  without  interrupting  the  current. 

CONDITIONS    OF    EXPERIMENT. 

Temperature  of  the  liquid :   This  must  be  from  50-60°. 

Current  density :  ND100  =  0.5-1  amp. 

Electrode  tension :  3.5—4.8  volts. 

Roughened  or  polished  dishes  answer  equally  well. 

Time :   About  2  hours. 

EXPERIMENT. 

Used  1.8-2  g  zinc  ammonium  sulphate,  4  g  potassium 
oxalate,  120  cc  liquid. 

CUrAmpSes8ity'     Electr^tsTension'         Temp.  Time.  Found.* 

0.5-0.55  3.5-4.0  55-60°  2  hr.  16.44# 

0.9-1  4.7-4.8  60°  1  "     50  m.  16.42 " 

According  to  v.  Miller  and  Kiliani,  4  g  potassium  oxa- 
late and  3  g  potassium  sulphate  are  dissolved  in  water,  the 
neutralized  zinc  solution  (sulphate  or  nitrate  containing  not 
more  than  0.3  g  Zn)  carefully  added,  and  electrolysis  effected 
without  heat,  by  a  current  of  ND100  =  0.3-0.5  amperes. 
The  reaction  is  complete  in  2  to  3  hours. 

N.  Eisenbergf  obtained  the  following  results  by  the  above 
method : 

Current         Electrode  Condition 

Density,         Tension,        Temp.  Time.  Found.  of 

•en-        Amperes.         Volts.  Metal. 

1.8312      0.4-0.35    3.95-4.00  25    -26°    4  hr.  16.35$  partly  spongy 

1.8312     0.40-0.35    4.15-4.25  28.5-30°    4  "     15  m.  16.01 "      spongy 
Remark:  (1)  Rough  dish;  (2)  Polished  dish. 

The  agitation  of  the  liquid  by  means  of  a  stirring  appli- 
ance is  recommended  for  this  method. 

According  to  Jordis,  zinc,  when  present  in  the  form  of 
sulphate,  chloride  or  nitrate,  may  be  separated  from  a  lactic 

*  [Theory  16.29^  Zn.]  \  Inaugural-Dissert.  Heidelberg,  1895. 


148        QUANTITATIVE   ANALYSIS   BY    ELECTROLYSIS. 

acid  solution.  The  ease  with  which  this  method  may  be  car- 
ried out  appears  from  the  directions  of  the  author,  which 
read  as  follows  :*  "  2  g  ammonium  sulphate  and  5-7  g  ammo- 
nium lactate  are  added  to  the  neutral  solution  containing  not 
less  than  0.3-0.5g  zinc,  and  it  is  acidified  with 'a  few  drops 
of  lactic  acid.  A  stirring  attachment  is  employed,  and  the 
solution  is  electrolyzed  with  a  current  of  !ND100  =  1.0-1.5 
amp.  After  40-60  minutes  the  electrolyte  is  poured  into  a 
second  dish  and  the  separation  completed  in  this.  With  a 
current  of  the  above  density  this  requires  20-25  minutes.  A 
somewhat  more  concentrated  solution  of  about  120-150  cc 
is  advantageous. 

4  c  Since  the  lactic  acid  is  but  very  slowly  decomposed 
during  the  electrolysis,  its  regeneration  resulting  from  the 
action  of  the  sulphuric  acid  formed  upon  the  ammonium 
lactate,  the  electrolyte  remains  acid  until  the  end  and  requires 
no  further  attention." 

MANGANESE. 

LITERATURE  I 

Riche,  Ann.  d.  China,  et  Pliys.,  [5],  13,  508. 

Luckow,  Zeit.  f.  anal.  Chem.,  19,  17. 

Schucht,  ibid.,  22,  493. 

Classen  and  v.  Reiss,  Ber.  deutsch.  chem.  Ges.,  14,  1622. 

Moore,  Chem.  News,  53,  209. 

Smith  and  Frankel,  Journ.  Anal.  Chem.,  3,  385  ; 

Chem.  News,  60,  262. 
Brand,  Zeit.  f.  anal.  Chem.,  28,  581. 
Riidorff,  Zeit.  f.  angew.  Chem.,  15,  6. 
Classen,  Ber.  deutsch.  chem.  Ges.,  27,  2060. 
Engels,  Zeit.  f.  Elektrochemie,  1895-96,  p.  413  ;  1896-97,  p.  286. 
Groeger,  Zeit.  f.  angew.  Chem.,  1895,  p.  253. 

*  Zeit.  f .  Elektrochemie,  1895-96,  p.  656 


MANGANESE.  149 

From  the  results  of  experience  in  the  Aachen  laboratory, 
none  of  the  methods  long  in  use  are  applicable  for  the  direct 
quantitative  determination  of  this  metal  as  peroxide.  It  is  gen- 
erally assumed  that  the  peroxide  when  dried  at  about  68°  has 
the  composition  MnOa.HaO,  an  assumption  which  the  author 
cannot  confirm.  If  the  attempt  be  made  to  convert  the 
hyd rated  peroxide  into  anhydrous  peroxide  by  prolonged 
drying  at  a  higher  temperature,  a  strongly  hygroscopic  sub- 
stance results  which  rapidly  increases  in  weight  during  the 
process  of  weighing.  It  is  therefore  necessary  to  convert  the 
dried  peroxide  into  mangano-manganic  oxide  by  ignition,  an 
operation  conducted  with  ease  and  safety.  After  determin- 
ing the  necessary  conditions  for  the  separation  of  large  quan- 
tities of  lead  peroxide,  the  author  was  induced  to  assume  that 
manganese  behaved  similarly  to  lead.  Investigation  proved, 
however,  that  strong  inorganic  acids  interfere  with  complete 
precipitation,  and  even  make  it  impossible.  Of  the  organic 
acids,  acetic  acid  alone  is  suitable,  although  the  precipitation 
of  large  quantities,  even  when  roughened  dishes  are  used, 
cannot  be  successfully  carried  out,  since  it  is  impossible  to 
obtain  firmly  adhering  precipitates. 

As  will  be  stated  under  lead,  the  separation  of  lead  takes 
place  from  nitric  acid  solutions  in  the  presence  of  other 
metals.  The  hope  that  manganese  in  the  presence  of  iron 
might  be  separated  and  determined  in  an  acetic  acid  solution 
has  not  been  fulfilled.  Innumerable  experiments,  conducted 
under  the  most  varied  conditions  and  with  the  most  diverse 
substances,  have  given  no  satisfactory  results.  In  view  of  the 
great  importance  which  a  method  for  the  direct  determina- 
tion of  manganese  in  the  presence  of  iron,  etc.,  would  pos- 
sess, this  investigation  will  be  continued. 


150         QUANTITATIVE   ANALYSIS   BY   ELECTROLYSIS. 


CONDITIONS    OF    EXPERIMENT. 

Temperature  of  the  liquid  :   50-70°. 
Current  density :  ND]00=  0.3-0.35  amp. 
Electrode  tension:   4.3-4.9  volts. 
Time:   3  hours. 

EXPERIMENT. 

Used  about  0.5  g  MnSO4.(NH4)aSO4.6HaO,  which  was 
dissolved  in  about  75  cc  water  and  eleetrolyzed  after  the  ad- 
dition of  25  cc  acetic  acid  sp.  g.  1.069  (20°). 

Current  Density,     Electrode 

Amperes.  Tension,          Temp.         Time.  Found. 

Volts. 

0.3-0.3  4.4-4.9        50-68°       3  hr.          0.1035 g  Mn3O4     14.94$* 

0.3-0.35          4.3-4.6         56-62°       3"  0.1045 g  Mn3O4     15-04'' 

An  equally  rapid  and  complete  separation  was  secured  by 
Engels,  as  a  result  of  investigations  conducted  in  the  Aachen 
laboratory.  The  method  is  as  follows  :  1-2  g  of  the  manga- 
nese salt  is  dissolved  in  about  125  cc  of  water,  and  10  g  am- 
monium acetate  and  1 . 5-2  g  chrome  alum  are  also  added. 
The  clear  solution  is  then  eleetrolyzed.  Chlorides  must  not 
be  present,  since  the  evolution  of  chlorine  interferes  with  the 
separation  of  the  manganese.  If  they  are  present,  the  pro- 
cess is  carried  out  according  to  the  directions  given  under  the 
separation  of  manganese  and  copper. 

CONDITIONS    OF    EXPERIMENT. 

Temperature  of  liquid :  80°. 
Current  density:   ND100=  0.6-1  amp. 
Electrode  tension  :   2.8-4  volts. 
Time :   About  1 J  hours. 
Note :  Roughened  dishes  must  be  used. 


*  [Theory  14.07$  Mn.     Probably  impure  salt  was  used. — Trans  ] 


MANGANESE.  151 


EXPERIMENT. 

In  the  determinations  given  below,  10  g  ammonium  ace- 
tate and  1.5-2  g  chrome  alum  were  added  to  the  solution. 

TT  \  /o/^  \    z-cr  r\      Current  Density    Electrode     m™,^  Timp  Found* 

Mn(NH4),(S04)a.6H,0.         NDloo,  Amp.         Tension,      Temp.  Time.  Mn,O4, 


en,  , 

Volts.  g.     Per  cent. 

1.1522g  0.6-0.5  2.8-3.1  80°  |br.  0.2235  19.39 

1.2554"  0.6-0.5  2.8-3.1  80°  "    "    0.2436  19.40 

1.2994"  0.6  3.  83°  "    "    0.2520  19.39 

1.8099"  1.1  3.7-4.1  80°  "    "   0.3513  19.40 

In  the  determination  of  manganese  in  the  salts  of  perman- 
ganic acid,  the  solution  of  the  latter  is  decomposed,  accord- 
ing to  Engels,  with  5  cc  acetic  acid  and  enough  hydrogen 
peroxide  to  completely  decolorize  it.  Since  the  presence  of 
even  small  quantities  of  hydrogen  peroxide  prevents  the  sepa- 
ration and  the  firm  adherence  of  the  precipitate,  the  excess 
of  hydrogen  peroxide  must  be  removed.  This  may  be  most 
easily  accomplished  by  the  addition  of  small  quantities  of 
chromic  acid,  until  further  addition  no  longer  causes  the  evo- 
lution of  gas;  generally  0.3-0.5  g  is  sufficient. 

EXPERIMENT. 

50  cc  of  a  potassium  permanganate  solution  were  decom- 
posed with  5  cc  acetic  acid  and  10  cc  of  a  weak  solution  of 
hydrogen  peroxide.  The  excess  of  HaO2  was  removed  with 
Cr03. 

Current  Density.               Tension.  Time.  Temp.  Mn3O4. 

I.            1.5  amp.  2.8    volts  1  hr.  85°  0.1217  g 

II.    "        1.65    "  3.15     "  1  "  85°  0.1220" 

III.            1.78    "  3.4      "  1  "  80°  0.1220" 

The  current  strength  available  varies  between  compara- 
tively wide  limits.  Weak  currents  also  give  rapid  and  satis- 
factory results. 

*  [Theory  19.52#  Mn3O4.] 


152         QUANTITATIVE  ANALYSIS   BY   ELECTROLYSIS. 

EXPERIMENT. 

Three  dishes,  each  containing  manganese  sulphate  solu- 
tion, 10  g  ammonium  acetate  and  1  g  chrome  alum,  were  con- 
nected in  parallel,  and  the  current  from  a  thermopile  passed 
through.  The  tension  at  the  electrodes  at  the  beginning  of 
the  electrolysis  was  3  .  2  volts,  the  entire  current  strength  1.5 
amp.,  so  that  each  dish  received  about  0.4  amp.  The  man- 
ganese salt  used  contained  20.45$  Mn8O4. 


Time.  Found. 

1.1955  0.22  3.2         80°       2hrs.  30  min.  20.45£ 

0.9009  0.22  3.2         80°      2   "     "     "  20.44" 

1.2012  0.22  3.2         80°      2   "     "     "  20.40" 

Since  manganese  separates  as  peroxide  from  a  cold  solution 
to  which  ammonium  acetate  has  been  added,  at  1.25  volts, 
and  when  warmed  to  80°  as  low  as  1-1.1  volts,  the  electroly- 
sis may  therefore  be  conducted  with  low  electromotive  forces. 
The  constancy  of  the  latter  may  be  assured  by  connecting  in 
shunt  (page  73).  The  lower  the  tension,  the  longer  the  time 
required  for  the  separation.  With  the  maximum  tension  of 
1.8  volts  it  takes  from  4  to  5  hours.  For  the  firm  adher- 
ence of  the  precipitate  a  temperature  of  80°  is  essential. 

In  those  cases  (i.e.,  in  the  presence  of  silver)  where  the 
chrome  alum  produces  a  precipitate  in  the  solutions,  it  may  be 
replaced  by  10  cc  of  alcohol,  which  in  general  is  not  as  effi- 
cient as  the  chrome  alum  for  separating  the  manganese  perox- 
ide. 

When  alcohol  is  used,  the  electrolysis  is  conducted  at  a 
temperature  of  75-80°,  with  a  maximum  tension  of  2  volts, 
which  gives  a  current  density  NDJOO  =  about  0.15  amp.  Time 
required  for  the  electrolysis,  about  5  hours. 


ALUMINIUM,  URANIUM,  CHROMIUM,  BERYLLIUM.     153 

ALUMINIUM,  URANIUM,  CHROMIUM,  BERYLLIUM. 

If  a  solution  of  aluminium  ammonium  oxalate  containing 
ammonium  oxalate  in  excess  is  submitted  to  the  action  of  the 
electric  current,  the  ammonium  oxalate  is  changed  into  car- 
bonate, and  the  aluminium  separates  as  hydroxide.  When  the 
oxalate  is  decomposed,  the  solution  is  heated  until  there  is  only 
a  faint  odor  of  ammonia,  the  hydroxide  filtered  off,  washed 
with  water,  and  converted,  by  ignition,  into  A12O3, 

Uranium  is  acted  on  in  the  same  way  as  aluminium. 

Chromium  ammonium  oxalate  is  oxidized  by  the  current 
with  formation  of  ammonium  chromate.  To  determine  the 
chromic  acid,  the  ammonium  carbonate  is  decomposed  by 
boiling,  the* solution  acidified  with  acetic  acid,  and  the  chromic 
acid  determined  as  lead  or  barium  chromate. 

When  beryllium  ammonium  oxalate  is  subjected  to  elec- 
trolysis, the  beryllium  is  kept  in  solution  by  the  hydrogen 
ammonium  carbonate  produced,  provided  the  solution  is  cold. 

The  behavior  of  aluminium,  chromium,  uranium,  and  be- 
ryllium can  be  made  use  of,  as  explained  later,  to  separate 
them  from  each  other  and  from  all  metals  which  separate 
from  their  double  oxalates  in  the  metallic  state  at  the  nega- 
tive electrode. 

COPPER. 

LITERATURE  : 

Gibbs,  Zeit.  f.  anal.  Chem.,  3,  334. 

Boisbaudran,  Bull.  d.  1.  Soc.  Chiin.,  1867,  p.  468. 

Merrick,  Amer.  Chemist,  2,  136. 

Wrightson,  Zeit.  f.  anal.  Chem.,  15,  299. 

Herpin,  ibid,,  15,  335. 

Ohl,  ibid.,  18,  523. 

Classen,  Ber.  deutsch.  chem.  Ges.,  14,  1622,  1627. 


154        QUANTITATIVE  ANALYSIS    BY    ELECTIIOLYS13. 

Classen  and  v.  Keiss,  Zeit.  f.  anal.  Chem.,  14,  246. 
Hampe,  Berg-  und  Hiittenm.  Ztg.,  21,  220  ;  25,  113. 
Riche,  Zeit.  f.  anal.  Chem.,  21,  116, 
Mackintosh,  Am.  Chem.  Journ.,  3,  354. 
Riidorff,  Ber.  deutsch.  chem.  Ges.,  21,  3050 ; 

Zeit.  f.  angew.  Chem.,  1892,  p.  5. 
Luckow,  Zeit.  f.  anal.  Chem.,  8,  23. 
Warwick,  Zeit.  f.  anorg.  Chem.,  1,  285. 
Smith,  Am.  Chem.  Journ.,  12,  329. 
Croasdale,  Journ.  of  Anal,  and  Appl.  Chem.,  5,  133. 
Foote,  Am.  Chem.  Journ.,  6,  333. 
Meeker,  Journ.  of  Anal,  and  Appl.  Chem.,  6.  267. 
Classen,  Ber.  deutsch.  chem.  Ges.,  27,  2060. 
Heidenreich,  ibid.,  29,  1585. 

Regelsberger,  Zeit.  f.  angew.  Chemie  (1891),  16,  473. 
Oettel,  Chemiker-Zeitung,  1894,  p.  879. 
Schweder,  Berg-  und  Huttenmann.  Ztg.,  36  (5)  11,  21. 

/  If  copper  be  reduced  from  a  solution  containing  an  excess 
of  ammonium  oxalate,  it  is  not  always  possible  to  obtain  the 
metal  in  a  compact  form.  For  this  reason  the  author,  as 
long  ago  as  1888,*  began  experiments  on  the  determination 
of  this  metal  from  a  solution  of  the  acid  double  oxalate. 
Further  experiments  in  this  direction  have  shown  that  coher- 
ent, bright  red  copper  precipitates  can  be  obtained  when  cop- 
per is  reduced  from  such  solutions  at  a  temperature  of  about 
80°.  The  solution  containing  the  copper  is  treated  with  a 
cold-saturated  solution  of  ammonium  oxalate,  heated  as  di- 
rected, and  at  first  electrolyzed  for  a  few  minutes  without  the 
addition  of  oxalic  acid.  A  cold- saturated  oxalic  acid  solu- 
tion is  then  run  in  from  a  burette.  The  method  of  proced- 
ure here  is  similar  to  that  described  under  Zinc  (page  147). 

In  the  analysis  of  substances  low  in  copper,  the  solution 
may  be  made  acid  at  the  start ;  in  concentrated  solutions,  on 
the  contrary,  the  electrolysis  must  be  conducted  in  solutions 

*Ber.  deutsch.  chem.  Ges.,  21,  2898. 


COPPER.  155 

which  are  as  nearly  neutral  as  possible,  since  otherwise  diffi- 
cultly soluble  oxalate  of  copper  will  separate  out,  owing  to 
the  free  oxalic  acid  present.  The  end  of  the  reaction  is  de- 
termined by  testing  with  potassium  ferrocyanide  a  small  por- 
tion of  the  solution  strongly  acidified  with  hydrochloric  acid. 
The  precipitate  must  be  washed  without  stopping  the  current. 
The  metal  is  dried  in  an  air-bath  after  treating  with  water 
and  alcohol. 

The  precipitated  copper  has  a  bright  red  color,  adheres 
firmly  to  the  dish,  and  has  little  resemblance  to  the  copper 
precipitated  from  nitric  acid  solutions  (see  below).  The 
chief  advantage  of  this  method  is  the  rapidity  with  which  it 
may  be  conducted. 

CONDITIONS    OF    EXPERIMENT. 

Temperature  of  the  liquid  :   80°. 

Current  density:,  JSD100  =  0.5-1  amp.  Most  favorable 
current  density  ND100  =  1  amp. 

Electrode  tension:   2.5-3.2  volts. 
Time  of  electrolysis;  2  hours. 

EXPERIMENT.  * 

Used  1  g  copper  sulphate,  4  g  ammonium  oxalate,  120  cc 

liquid. 

CUrAmp?reensSity'      Elect^Jensioa<     Temp.        Time.           Found.  Taken. 

1.0-0.8.               2.8-3.2               80°        2  hr.        0.2531  g  0.2529  g 

0.45-0.35             2.5-2.8               80°        2J  "         0.2528 "  0.2529 " 
Copper  precipitate  bright  red. 

As  has  been  observed  by  Luckow,  copper  may  also  be 
precipitated  from  a  solution  to  which  nitric  acid  has  been 
added. 

*  Separation  of  copper  from  a  solution  of  the  ammonium  double 
oxalate  in  the  presence  of  free  oxalic  acid. 


156        QUANTITATIVE  ANALYSIS   BY    ELECTROLYSIS. 

The  reduction  of  copper  from  a  nitric  acid  solution  de- 
pends upon  the  presence  of  a  certain  quantity  of  nitric  acid 
and  the  absence  of  chlorides.  To  about  200ccof  solution, 
containing  the  copper  as  sulphate,  20  cc  of  nitric  acid  *  (sp. 
g.  =  1.21)  are  added  and  the  liquid  is  subjected  to  electrolysis. 
The  end  of  the  reaction  is  determined  with  ammonia. 

The  presence  of  chlorides  is  to  be  avoided.  In  the  pres- 
ence of  antimony,  arsenic,  mercury,  silver,  tin,  and  bismuth, 
traces  of  these  metals  come  down  with  the  copper ;  but  iron, 
cobalt,  nickel,  cadmium,  manganese,  and  zinc  can  be  separated 
from  copper  by  this  method. 

According  to  the  researches  of  Schroder  large  quantities 
of  iron  are  detrimental,  since  a  secondary  reaction  may  take 
place  between  the  ferric  salt  formed  and  the  precipitated  cop- 
per, which  causes  the  copper  to  redissolve. 

Copper  separates  in  a  crystalline  form  from  solutions 
warmed  to  50-60° ;  it  is  nevertheless  impossible  to  separate 
the  last  traces  of  copper  at  this  temperature. 

CONDITIONS    OF    EXPERIMENT. 

Temperature  of  solution :  20-30°. 

Current  density:  ND100  =  0.5-1  amp.  The  latter  only 
when  no  other  metal  than  copper  is  present  in  the  solution. 

Electrode  tension  :   2.2-2.5  volts. 

Time  :  4-5  hours.  Agitating  the  solution  with  a  stirring 
attachment  hastens  the  operation. 

EXPERIMENT. 

Used  about  1  g  copper  sulphate  and  5$  by  volume  nitric 
acid.  Entire  volume  of  liquid  120  cc. 

*  Such  a  large  quantity  of  nitric  acid  is  required  only  when  the  separa- 
tion of  copper  from  other  metals  is  to  be  carried  out.  If  no  other  metal 
than  copper  is  present  in  the  solution,  2  or  3  per  cent  by  volume  of  nitric 
acid  is  sufficient. 


COPPER.  157 

Electro^Jtesnsion'    Temp.       Time.       Found.  Taken. 

1.1-1.0  2.2-2.5       25-30°     5  hr.      0.2490  g  Cu  0.2495  gCu 

1.0-0.95  2.25-2.3       30-32°     5"        0.2505""      0.2510"" 

A  solution  containing  free  nitric  acid  may  also  be  used 
for  separating  such  metals  as  are  not  reduced  in  the  presence 
of  this  acid,  or  which  are  set  free  at  the  positive  electrode  in 
the  form  of  peroxides.  In  such  cases,  however,  it  must  be 
kept  in  mind  that  the  nitric  acid  is  gradually  converted  into 
ammonia,  on  account  of  which,  after  the  current  has  acted 
for  some  time,  nitric  acid  must  be  occasionally  added. 

Copper  may  be  separated  from  a  solution  containing  am- 
monium oxalate  or  one  containing  free  nitric  acid,  in  the 
presence  of  small  quantities  of  antimony  and  arsenic.  If, 
however,  the  amounts  of  the  latter  are  considerable,  then, 
after  continued  action  of  the  current,  antimony  and  arsenic 
are  deposited  upon  the  copper,  causing  the  negative  electrode 
to  appear  more  or  less  dark-colored.  In  order  to  determine 
the  copper  in  such  cases,  the  dried  electrode  is  ignited  for  a 
short  time,  as  a  result  of  which  the  copper  is  oxidized  and 
the  antimony  and  arsenic  are  driven  off.  The  residue  of 
oxide  is  dissolved  in  nitric  acid  and  again  submitted  to  elec- 
trolysis.* 

In  general  the  presence  of  chlorides  causes  the  copper  to 
separate  in  a  spongy  condition.  To  avert  this  action  and  to 
secure  an  adherent  precipitate,  Riidorff  adds  2-3  g  ammonium 
nitrate  and  20  cc  ammonia  (sp.  g.  0.96),  dilutes  with  water  to 
100  cc,  and  electrolyzes  this  solution.  At  the  close  of  the  re- 
duction the  solution  is  acidified  with  dilute  acetic  acid,  the 
dish  filled  to  overflowing  with  water,  emptied,  shaken  to  re- 
move the  last  drops  of  water,  and  dried  at  100°  in  the  air- 
bath. 

In  the  laboratory  of  the  Technical  High  School  at  Munich 
*  Mansfeld'scbe  Hiittendirection. 


158         QUANTITATIVE  ANALYSIS   BY   ELECTROLYSIS. 

the  preceding  method  is  carried  out  under  the  following  con- 
ditions :  Ammonia  is  added  in  slight  excess  until  the  precipi- 
tate which  at  first  appears  is  redissolved.  Then  20-2 5  cc 
ammonia,  sp.  g.  0.96,  are  added,  in  case  not  more  than  0.5g 
copper  is  present.*  In  this  solution  3-5  g  ammonium  nitrate 
are  dissolved  and  the  electrolysis  is  conducted  with  a  current 
of  OT)JOU  —  2  amperes.  The  precipitate  must  he  washed 
without  interrupting  the  current. 

Oettel,  who  also  carried  out  experiments  on  the  quantita- 
tive determination  of  copper  from  ammoniacal  solutions, 
found  that  by  the  addition  of  ammonium  nitmte  0.2-0. 25  g 
of  copper  sulphate  were  quantitatively  reduced  in  6-8  hours 
at  ordinary  temperatures.  The  results  of  his  investigation  are : 

"  1 .  That  copper  can  be  separated  in  a  compact  form 
from  weakly  ammoniacal  solutions  containing  ammonium 
nitrate,  by  currents  of  ]STD100  =  0.07-0.27  ampere.  With  too 
little  ammonium  nitrate,  as  well  as  in  the  presence  of  large 
quantities  of  free  ammonia,  the  precipitate  shows  a  tendency 
to  a  spongy  structure. 

"2.  The  highest  concentration  of  the  solution  is  0.8  g 
copper  per  100  sq.  cm.  electrode  surface,  with  the  employ- 
ment of  a  wire-shaped  positive  electrode. 

"3.  The  presence  of  chlorine,  zinc,  arsenic,  and  small 
quantities  of  antimony  is  without  detrimental  action  ;  when  the 
solution  contains  lead,  bismuth,  mercury,  cadmium,  or  nickel, 
the  results  of  the  determinations  are  somewhat  too  high." 

E.  F.  Smith  f  precipitates  copper  from  a  solution  which 
contains  sodium  phosphate  and  free  phosphoric  acid. 
Heidenreich,  who  tested  the  method  in  the  Aachen  laboratory, 
obtained  the  following  results. 

*  If  as  much  as  1  g  Cu  is  present,  the  quantity  of  ammonia  is  increased 
to  30-35  cc. 

f  Electrochemical  Analysis,  p.  92. 


COPPER.  159 

EXPERIMENT. 

100  cc  of  a  solution  of  NaJIPO4  (1.0358  g)  and  3.o  cc 
of  a  solution  of  phosphoric  acid  (1.347  g)  were  diluted  with 
water  to  110  cc.  To  this  solution  the  copper  solution  was 
added. 

Taken.  Volts.  Time.  Found.* 

0.3959  g  2.4-2.6  17  hr.  25.41  £ 

0.3982  "  2.4-2.6  17  "  25.26 " 

The  copper  separated  at  first  brilliantly  metallic,  but  as 
the  electrolysis  proceeded  it  became  dark  red  and  spongy. 
Variations  of  the  conditions,  such  as  increasing  the  tension, 
led  to  no  better  results.  Owing  to  the  spongy  condition  of 
the  precipitate,  the  results  came  too  high. 

For  the  special  determination  of  copper,  in  copper-alumin- 
ium alloys,  liegelsberger  suggests  dissolving  3-5  g  of  the 
alloy  in  nitric  acid  and  evaporating  the  solution  down  to  the 
consistency  of  sirup.  The  sample  is  diluted,  and  a  measured 
quantity  (corresponding  to  0.6-1  g  substance)  is  poured  into 
the  electrolytic  cell.  An  excellent  precipitate  is  obtained  if 
the  acid  solution  is  neutralized  with  ammonia  and  10  cc  of 
dilute  nitric  acid  (sp.  g.  1.2)  are  added  to  200  cc  of  the 
liquid.  The  clear  solution  is  electrolyzed  with  a  current  den- 
sity NDIOO  —  OA  amp.  When  the  solution  is  warmed  the 
separation  is  completed  in  about  three  hours. 

A  rapid  and  accurate  method  for  the  determination 
of  copper  has  been  worked  out  by  Carl  Engels  in  the 
Aachen  laboratory.  This  method  has  the  advantage  over  the 
use  of  nitric  acid  solutions  that  it  can  be  more  rapidly  per- 
formed, and  that,  in  separations,  it  also  dispenses  with  the 
tedious  conversion  of  the  nitrates  into  sulphates.  This  method 
is  based  upon  the  addition  of  urea. 

The  separation  of  copper  from  solutions  containing  sul- 
*  [Theory  25.33#  Cu.] 


160        QUANTITATIVE  ANALYSIS   BY    ELECTKOLYSIS. 

phuric  acid  is  possible  also  if  hydroxylamine  be  added.     The 
method  is  as  follows : 

If  the  separation  is  to  be  carried  out  with  weak  currents, 
say  during  the  night,  the  addition  of  2  cc  concentrated  sul- 
phuric acid  and  about  J-  g  hydroxylamine  sulphate  is  recom- 
mended. A  fine  crystalline  precipitate  and  absolutely  accurate 
results  are  obtained  with  a  current  strength  of  OT3100  = 
0.08-0.18  ampere.  The  tension  at  the  poles  of  a  shunt  cir- 
cuit was  1.8-2.2  volts;  after  connecting  the  dish  the  tension 
sank  to  1.1-1.3  volts,  with  a  current  of  0.1-0.2  amp. 

EXPERIMENT. 

Taken  Current  Density       Tension,         T,  Found.          p  ^ 

CuS04.5H2O.  'ND100.  Volts.  me'  Cu. 

1.0130g.  0.1   amp.  1.1  Night  0.2574  25.41 

1.7065  "  0.12     "  1.3  "  0.4335  25.40 

1.1893  "  0.1       "  1.2  "  0.3021  25.41 

If  stronger  currents  are  used,  the  amount  of  sulphuric 
acid  must  be  increased.  10—15  cc  of  cone,  sulphuric  acid 
are  poured  into  the  solution  of  the  salt,  it  is  diluted  to  150 
cc,  and  1  g  hydroxylamine  sulphate  is  added.  If  0.3-0.5 
g  Cu  is  present,  with  a  current  strength  of  NDJOO  —  1  amp., 
the  separation  is  finished  in  1^  to  2  hours.  The  condition  of 
the  precipitated  copper  is  much  better  and  much  more  suited 
for  quantitative  determination  than  the  copper  obtained  under 
similar  conditions  without  the  addition  of  hydroxylamine. 

Urea  exerts  a  far  more  satisfactory  action  than  hydroxyl- 
amine upon  the  separation  of  copper  from  solutions  contain- 
ing sulphuric  acid.  "With  a  current  strength  of  OT)]00  =  1 
ampere,  not  the  slightest  tendency  toward  a  spongy  separa- 
tion is  exhibited,  but  a  bright-red  crystalline  coating  is 
obtained  on  the  negative  electrode.  The  analysis,  with  the 
stated  current  density,  is  completed  in  1J  hours. 

10-15  cc  concentrated  sulphuric  acid  and  1  g  urea  are 
*  [Theory  25.33$  Cu.] 


.;urreui  ueusity 
NDjooi  Amp. 

JLVU81OU, 

Volts. 

Temp. 

Time. 

Found.* 

1.05 

3.1 

25° 

1  hr.  15  m. 

25.09  % 

1.2 

3.1 

55° 

1   "   15   " 

25.09" 

0.75 

2.7 

65° 

1  "  45  " 

25.09" 

COPPER.  161 

added  to  the  solution  of  the  copper,  which  is  then  diluted  to 
150  cc. 

CONDITIONS    OF    EXPERIMENT. 

Temperature  of  liquid  :  Most  suitable,  60-70°. 
Electrode  tension :   2.7-3.1  volts. 
Current  density:  KD100  =  0.8-1  amp. 
Time  :   1 J-  hours. 

EXPERIMENT. 

Used  CuSO4.5H2O. 

Quantsub 

1.1364 
0.9671 
1.3972 

The  current  may  be  interrupted  in  washing  the  precipitate. 
The  separated  copper  contains  traces  of  carbon,  and  also  plat- 
inum which  dissolves  from  the  anode.  These  admixtures 
can  be  determined  by  dissolving  the  copper  in  dilute  nitric 
acid  (1  :  10).  A  thin  dark  coating  remains  on  the  dish, 
which  may  be  washed  with  water,  but  not  with  alcohol,  with- 
out becoming  loosened.  The  weight  of  the  dish,  determined 
after  washing  and  drying  in  the  air-bath,  is  used  as  a  basis 
for  calculating  the  weight  of  the  separated  copper. 

With  weaker  currents  the  length  of  time  required  is  of 
course  greater.  With  a  current  density  of  ND100  =  0.2  am- 
pere, the  precipitation  of  from  0.3  to  0.4  g  Cu  is  completed 
in  3£- 4  hours.  It  is  desirable  in  this  case  to  add  less  sul- 
phuric acid  to  the  solution;  5  cc  cone.  HaSO4  to  each  150 
cc,  is  the  proper  proportion. 

Four  dishes  were  connected  in  parallel,  and  for  every  150 
cc  of  solution  of  the  copper  salt  which  they  contained  1  g 
urea  and  5  cc  cone.  H,SO4  were  added.  The  four  electroly- 
*  [Theory  25.33^] 


162         QUANTITATIVE   ANALYSIS   BY  ELECTROLYSIS. 

ses  were  then  conducted  in  the  cold,  with  the  current  from 
a  thermopile.  The  entire  current  strength  was  !ND100  =  0.8 
ampere,  so  that  each  dish  received  a  current  of  ND100  =  0.2 
ampere.  The  analyses  were  completed  in  4  hours.  The  per 
cent,  of  copper  in  the  salt  used  was  25.08. 

Used  CuSO4.5H2O.  Found  Cu.  Found  %. 

l.OlOlg  0.2533g  25.07 

1.0815  "  0.2709  "  25.05 

1.0320  "  0.2589  "  25.08 

1.0111  "  0.2535  "  25.07 

BISMUTH. 

LITERATURE  : 

Luckow,  Zeit.  f.  anal.  Chem.,  19,  16. 

Classen  and  v.  Reiss,  Ber.  deutsch.  chem.  Ges.,  14,  1622. 

Thomas  and  Smith,  Am.  Chem.  Journ.,  5,  114. 

Moore,  Chem.  News,  53,  209. 

Smith  and  Knerr,  Am.  Chem.  Journ.,  8,  206. 

Schucht,  Zeit,  f.  anal.  Chem.,  22,  492. 

Eliasberg,  Ber.  deutsch.  chem.  Ges. ,  19,  326. 

Brand,  Zeit,  f.  anal.  Chem.,  28,  596. 

Vortmann,  Ber.  deutsch.  chem.  Ges.,  24,  2749. 

Kiidorff,  Zeit.  f.  angew.  Chem.,  1892,  p.  199. 

Smith  andi  Saltar,  Zeit.  f.  anorg.  Chem.,  3,  418. 

Smith  and  Moyer,  Journ.  of  the  Am.  Chem.  Soc.,    15,  28,  101. 

Smith  and  Knerr,  Am.  Chem.  Journ.,  8,  206. 

Schmucker,  Zeit.  f.  anorg.  Chem.,  5,  199. 

Up  to  the  present  time  it  has  been  found  impossible  to 
quantitatively  precipitate  bismuth  in  a  compact  metallic  form. 
It  separates  in  a  more  or  less  spongy  form  from  all  its  com- 
pounds. A  discussion  of  the  directions  given  by  the  different 
investigators  will  therefore  be  omitted. 

G.  Vortmann  has  attempted  to  separate  bismuth  as  an 
amalgam.  Since,  however,  the  directions  for  the  conditions 
of  experiment  are  not  given,  the  mere  mention  of  this  method 
will  be  sufficient. 


CADMIUM.  163 


CADMIUM. 

LITERATURE  : 

Smith,  Am.  Phil.  Soc.  Pr.,  1878. 

Clarke,  Zeit.  f.  anal.  Chem.,  18,  104. 

Beilstein  and  Jawein,  Ber.  deutsch.  chem.  Ges.,  12,  759. 

Smith,  Am.  Chem.  Journ.,  2,  43. 

Luckow,  Zeit.  f.  anal.  Chem.,  19,  16. 

Wrightson,  ibid.,  15,  303. 

Classen  and  v.  Reiss,  Ber.  deutsch.  chem.  Ges.,  14,  1638, 

Warwick,  Zeit.  f.  anorg.  Chem.,  1,  258,  291. 

Moore,  Chem.  News,  53,  209. 

Smith,  Am.  Chem.  Journ.,  12,  329. 

Vortmann,  Ber.  deutsch.  chem.  Ges.,  24,  2749. 

Eiidorff,  Ztschr.  f.  angew.  Chem.,  1892. 

Classen,  Ber.  deutsch.  chem.  Ges.,  27,  2060. 

Heidenreich,  ibid.,  29,  1586. 

The  separation*  of  this  metal  in  a  compact  brilliant  form 
has  been  shown,  by  experiments  carried  out  at  the  Aachen 
laboratory,  not  to  be  possible  by  any  of  the  methods  hitherto 
described.  It  may  bo  accomplished,  however,  by  the  elec- 
trolysis of  a  warm  solution  of  the  double  oxalate  which  is 
kept  acid  with  oxalic  acid  during  the  electrolysis.  (A  cold- 
saturated  solution  of  oxalic  acid  is  employed.)  (See  direc- 
tions for  Zinc.) 

To  prepare  the  double  salt,  the  cadmium  compound  is 
dissolved  in  20-25  cc  water,  by  warming  in  a  platinum 
dish ;  a  hot  solution,  which  should  be  previously  filtered,  of 
10  g  ammonium  oxalate  in  80-100  cc  water  is  added  and  the 
solution  is  electrolyzed.  As  soon  as  the  action  of  the  current 
has  begun,  several  cubic  centimeters  of  oxalic  acid  are  poured 
upon  the  watch-glass  covering  the  dish,  and  the  liquid  is  kept 

*  The  metallic  condition  of  the  precipitated  cadmium,  all  the  conditions 
of  the  experiment  being  preserved,  depends  upon  the  absolute  cleanliness 
of  the  surface  of  the  cathode. 


164        QUANTITATIVE   ANALYSIS   BY    ELECTROLYSIS. 

weakly  acid  during  the  electrolysis.  The  end  of  the  reaction 
is  determined  with  hydrogen  sulphide,  by  testing  a  small  por- 
tion of  the  solution  acidified  with  hydrochloric  acid.  The 
metal  must  be  washed  without  interrupting  the  current. 
(Method  of  the  author.) 

CONDITIONS    OF    EXPERIMENT. 

Temperature  of  the  liquid  :  70-75°. 
Current  density:  !ND100  =  0.5-1  amp. 
Electrode  tension:  3-3.4  volts. 
Period  of  electrolysis :  About  3  hours. 
Maximum  quantity  of  metal  which  can  be  precipitated : 
0.15-0.16  g. 

EXPERIMENT. 

Used  0.3-0.4  g  cadmium  sulphate,  10  g  ammonium  oxa- 
late,  120  cc  liquid. 

CUTmpereenSSity'  Electr^oeltTsension'  Temp.  Time.  Found. 

0.6-0.5  2.75-3.4        73-76°        3  hr.  30m.  0.1472  g    49.07  #* 

1.0-0.8  3.0-3.4        68-73°        3"  0.1474  g    49.13" 

Precipitate  brilliantly  lustrous. 

"With  regard  to  further  experiments  by  this  method  and 
for  data  on  the  use  of  polished  and  roughened  dishes,  of  which 
the  former  are  in  this  case  to  be  preferred,  the  dissertation  of 
N.  Eisenberg  f  should  be  consulted. 

Smith  and  Luckow  recommend  the  precipitation  of  cad- 
mium from  a  solution  of  the  chloride  or  sulphate,  which  has 
been  saturated  with  sodium  acetate.  Eliasberg,  who  tested 
this  method  in  the  Aachen  laboratory,  found  that  the  reduc- 
tion took  place  readily  when  the  solution,  of  about  100  cc 
volume,  was  treated  with  about  3  g  sodium  acetate  and  a  few 

*[The  salt  taken  was  probably  CdSO4.H2O  containing  49.67$  Cd.— Trans.] 
f  Eisenberg,  Inaugural-Dissertation,  Heidelberg,  1895 


CADMIUM.  165 

drops  of  acetic  acid,  and  the  electrolysis  wab  carried  out  at  a 
temperature  of  40-50°. 

In  the  laboratory  of  the  Munich  High  School  the  forego- 
ing method  is  practised  as  follows :  The  solution,  neutralized  if 
necessary,  containing  not  more  than  0.5  g  cadmium,  is  treated 
with  3  g  sodium  acetate,  and  made  weakly  acid  with  acetic 
acid.  The  solution  is  warmed  to  45°,  and  decomposed  with 
a  current  of  ]SD10o  —  0.02-0.07  ampere.  The  metal  is 
washed  without  interrupting  the  current,  and  quickly  dried 
at  100°. 

During  the  electrolysis  the  solution  should  not  be  warmed 
above  50°,  on  account  of  the  formation  of  basic  salts.  Cad- 
mium is  only  partly  precipitated  from  solutions  strongly 
acidified  with  acetic  acid.  By  this  method  0.2  g  of  cadmium 
may  be  separated  in  about  five  hours.  The  presence  of 
nitrates  is  detrimental. 

According  to  Beilstein  and  Jawein,  the  determination  of 
cadmium  may  be  conducted  from  a  solution  of  the  double 
salt  with  potassium  cyanide.  Aside  from  the  fact  that  the 
necessary  directions  are  not  given,  this  method  possesses  no 
advantages,  the  precipitation  of  0.2  g  cadmium  requiring 
about  12  hours. 

Vortmann  attempted  the  determination  of  cadmium  by  a 
method  similar  to  that  used  for  the  determination  of  bismuth 
and  zinc,  by  precipitation  from  a  solution  of  the  ammonium 
double  salt  in  the  form  of  amalgam. 

E.  F.  Smith  determines  cadmium  by  dissolving  the  oxide 
in  acetic  acid,  evaporating,  taking  up  in  water,  and  electro- 
lyzing  the  solution  thus  obtained.  Heidenreich,  who  carried 
out  in  the  Aachen  laboratory  a  series  of  varied  researches  on 
this  subject,  obtained  no  satisfactory  results,  either  in  the 
condition  of  the  precipitate  or  in  the  quantitative  separation 
of  the  metal. 


166        QUANTITATIVE  ANALYSIS   BY   ELECTROLYSIS." 

A  further  method  by  E.  Smith  depends  upon  the  reduc- 
tion of  cadmium  from  a  solution  to  which  sodium  phosphate  and 
free  phosphoric  acid  have  been  added.  Here  also  the  quanti- 
tative separation  of  the  cadmium  does  not  take  place,  not  even 
when  the  current  is  increased  to  1  ampere. 

Finally,  Smith  *  has  proposed  a  method  for  the  separation 
of  cadmium  from  a  solution  containing  free  acetic  acid. 
According  to  the  statements  of  Heidenreich,  cadmium  is  pre- 
cipitated from  a  solution  containing  10  cc  acetic  acid  (50$) 
to  120  cc  of  solution,  by  a  current  density  of  0.4  ampere 
and  a  tension  of  4.5  volts,  in  the  form  of  small  crystalline 
plates  which  cannot  be  washed  without  loss.  Variations  of 
the  conditions  of  experiment  (addition  of  less  acetic  acid 
[2-10  cc],  employment  of  current  densities  of  from  0.1  to 
0.4  ampere  and  tensions  of  4-7. 5  volts,  as  well  as  different 
temperatures)  gave  no  satisfactory  results. 


LEAD.  * 
LITERATURE  : 

Kiliani,  Berg-  u.  Hiittenin.-Zeitung,  1883,  p.  253. 
Luckow,  Zeit.  f.  anal.  Chem.,  19,  215. 

Kiche,  Ann.  d.  Chim.  et  Phys.,  13,  508;  Zeit.  f.anal.  Chem.,  21,  117. 
Classen,  Zeit.  f.  anal.  Chem.,  21,  257. 
Hampe,  Zeit.  f.  anal.  Chem.,  13,  183. 

Parodi  and  Mascazzini,  Ber.  deutsch.  chem.  Ges.,  10,  1098; 
Zeit.  f.  anal.  Chem.,  16,  469;  18,  588. 
Kiche,  Zeit.  f.  anal.  Chem,,  17,  219. 
Schucht,  Zeit.  f.  anal.  Chem.,  21,  488. 
Tenny,  Am.  Chem.  Journ.,  5,  413. 
Smith,  Am.  Phil.  Soc.  Pr.,  24,  428. 
Vortmann,  Ber.  deutsch.  chem.  Ges.,  24,  2749. 
RMorff,  Zeit.  f.  angew.  Chem.,  1892,  p.  198. 

*  Electrochemical  Analysis,  p.  95. 


LEAD.  167 

Warwick,  Zeit.  f.  anorg.  Chem.,  1,  258. 

Classen,  Ber.  deutsch.  chem.  Ges.,  27,  163. 

Kreichgauer,  ibid.,  27,  315;  Zeit.  f.  anorg.  Chem.,  9,  89. 

Classen,  Ber.  deutsch.  chem.  Ges.,  27,  2060. 

Medicus,  ibid.,  25,  2490. 

Neumann,  Chemiker-Zeitung,  1896,  p.  381. 

If  a  solution  of  a  lead  salt  containing  an  excess  of  ammo- 
nium oxalate  be  electrolyzed  warm,  the  lead  separates  at 
the  negative  electrode,  adheres  closely,  and  shows  its  char- 
acteristic metallic  properties ;  but  it  oxidizes  partially  on  wash- 
ing with  water  and  alcohol,  so  that  the  results  are  always  too 
high.  The  precipitation  of  lead  as  amalgam  presents  some 
difficulties,  inasmuch  as  some  lead  peroxide  separates  at  the 
positive  electrode  and  must  be  dissolved.  According  to  G. 
Yortmann,  the  aqueous  solution  of  the  lead  salt,  containing 
sufficient  mercuric  chloride  to  produce  the  amalgam,  is  treated 
with  3-5  g  sodium  acetate  and  a  few  cubic  centimeters  of 
concentrated  potassium  nitrite  solution.  The  precipitate  pro- 
duced by  the  latter  reagent  (which  is  added  to  prevent  the 
formation  of  peroxide)  is  dissolved  in  acetic  acid,  and  the 
clear  yellow  solution  diluted  and  electrolyzed.  If  lead  per- 
oxide appears  on  the  positive  electrode  during  the  reaction, 
more  potassium  nitrite  is  added.  The  close  of  the  reaction  is 
determined  by  testing  with  ammonium  sulphide.  As  lead 
amalgam  oxidizes  rather  readily  when  moist,  it  is  quickly 
washed  with  water,  alcohol,  and  ether,  dried  by  the  warmth 
of  the  hand  and  by  blowing  upon  it,  and  finally  in  the  desic- 
cator. 

The  amalgam  may  also  be  separated  from  an  aqueous  solu- 
tion acidified  with  nitric  acid.  However,  as  free  nitric  acid 
favors  the  formation  of  lead  peroxide,  more  frequent  addition 
of  potassium  nitrite  is  necessary,  and  complete  precipitation 
is  thereby  seriously  hindered. 


168         QUANTITATIVE  ANALYSIS   BY   ELECTROLYSIS. 

In  a  solution  containing  free  nitric  acid,  lead  is  acted  on 
like  manganese;  it  is  oxidized,  and  separates  as  hydrated 
peroxide  at  the  positive  electrode.  If  there  is  no  other  metal 
in  the  solution,  it  must  contain  at  least  10  per  cent  free  nitric 
acid,  according  to  Luckow ;  in  the  presence  of  other  metals 
{mercury,  copper,  etc.),  the  oxidation  is  complete  even  in 
presence  of  little  nitric  acid. 

In  the  Munich  laboratory  experiments  have  been  con- 
ducted as  to  the  quantity  of  nitric  acid,  sp.  gr.  1.36,  and  have 
demonstrated  that  this  depends  on  the  temperature  and 
the  current  density  which  is  used.  The  current  density 
depends  in  turn  on  the  condition  of  the  surface  of  the  positive 
electrode.  If  this  is  very  smooth,  a  current  of  ND100  =  0.05 
is  sufficient,  otherwise  one  of  NDi00  =  0.5  is  needed  to  pro- 
duce an  adherent  precipitate.  When  ND]00  —  0.05  ampere, 
2  per  cent  by  volume  of  nitric  acid  should  be  added  when  the 
solution  is  heated,  and  10  per  cent  by  volume  at  ordinary 
temperatures.  "When  ED100  =  0.5  the  vohime-percentages 
are,  respectively,  7  and  20  for  heated  and  cool  solutions. 

Heating  the  solution  to  about  50°  materially  assists  the 
separation.  '  The  precipitate  may  be  washed  without  loss,  after 
the  current  is  interrupted. 

Chlorine  compounds  urns*,  not  be  present  in  the  solution 
for  electrolysis. 

Even  when  the  stated  conditions  are  observed,  the  quan- 
tity of  lead  which  can  be  precipitated  as  peroxide  in  an 
adherent  form  is  relatively  small.*  The  rapid  separation  of 
large  quantities  of  lead  peroxide,  firmly  adherent  like  a  metal, 
may  only  be  carried  out  without  difficulty,  as  the  author's 

*  From  experience  in  the  Aachen  laboratory,  the  greatest  possible  quan- 
tity is  0.15  gPbO2  per  100  sq.  era.  surface,  while  according  to  the  statements 
of  Dr.  Cohen  (Chem.  Ztg.,  1893,  No.  98)  as  much  as  0.3  g  can  be  precip- 
itated. 


LEAD.  169 

researches  have  shown,  when  the  inside  of  the  platinum  dish 
serving  as  anode  is  roughened  with  a  sand-blast.*  By  the 
use  of  such  dishes,  it  is  possible,  with  a  current  of  1.5  ampere, 
to  precipitate  in  a  few  hours  as  much  as  4  g  of  lead  peroxide 
on  100  sq.  cm.  of  surface. 

For  conducting  the  determination  of  lead,  after  the  solu- 
tion of  the  lead  salt  has  been  accomplished,  20  cc  nitric  acid 
(sp.  g.  1.35-1.38)  are  added,  the  solution  is  diluted  to  about 
100  cc,  warmed  to  60-65°,  and  electrolyzed  with  a  current 
of  ND100  =  1.5-1.7  amperes.  If  the  warming  is  continued 
during  the  electrolysis,  the  precipitation  of  quantities  up  to 
1 . 5  g  lead  peroxide  is  completed  in  about  3  hours ;  with  larger 
quantities  in  about  4-5  hours.  Complete  precipitation  is 
insured  by  adding  about  20  cc  of  water  and  observing 
whether  the  freshly  wetted  surface  of  the  electrode  becomes 
darker.  Incase  no  blackening  is  observed  at  the  end  of  10-15 
minutes,  the  current  is  stopped,  and  the  precipitate  is  washed 
with  water  and  alcohol,  and  dried  at  180-190°.  The  residue 
is  anhydrous  peroxide. 

CONDITIONS    OF    EXPERIMENT. 

Temperature  of  the  liquid:    60-65°. 

Current  density:    NDJOO  =  1.5—1.7  amp. 

Electrode  tension:  2.5  volts.  The  tension  is  without 
influence  upon  the  condition  of  the  peroxide,  and  may  vary 
within  wide  limits. 

EXPERIMENT. 

Used  Pb  (NO3)a  dissolved  in  100  cc  water,  with  the  ad- 
dition of  20  cc  nitric  acid  (sp  g.  1.35-1.38). 

*  The  platinum  refinery  of  G.  Siebert  in  Hanau  has  faultlessly  carried 
out  the  roughening  in  the  desired  manner  at  the  request  of  the  author. 
Such  roughened  dishes  are  of  course  applicable  to  all  other  electrolytic 
determinations. 


170        QUANTITATIVE   ANALYSIS   BY   ELECTROLYSIS. 


Current  Density, 
Amperes. 

Electrode  Tension, 
Volts. 

Temp. 

Time. 

Found.* 

1.55-1.45 

2.43-2.4 

60-65° 

1  hr.    5  m. 

72.20  % 

1.6  -1.58 

2.48-2.43 

60-65° 

1    "  10    " 

72.19" 

1.6  -1.65 

2.41-2.36 

60-65° 

1    "     5    " 

72.20" 

The  preceding  method  permits  of  the  separation  of  lead 
from  zinc,  iron,  nickel,  cobalt,  manganese,  copper,  cadmium, 
gold,  mercury,  antimony,  and  aluminium ;  in  the  presence  of 
silver  and  bismuth,  traces  of  these  metals  in  the  form  of 
peroxides  pass  over  into  the  lead  peroxide. 

THALLIUM. 

Tins  metal  may  be  completely  precipitated  from  an  am- 
monium oxalate  solution. 

The  properties  of  thallium, 
however,  are  similar  to  those  of 
lead ;  its  determination  therefore 
requires  special  consideration. 

G.  Neumann,  in  connection 
with  a  research  on  certain  double 
salts  of  thallium  in  the  Aachen 
laboratory,  has  also  investigated 
the  quantitative  determination  of 
the  metal.  As  his  method  is  of 
value  in  the  investigation  of 
thallium  compounds,  it  is  here 
described.  The  process  is  based 
on  precipitation  of  the  thallium 
as  metal,  and  determination  of 
the  volume  of  hydrogen  set  free 
by  its  solution  in  hydrochloric 
acid. 

The  apparatus  shown  in  Fig. 
K  is  a  flask  of  about  100  cc 


FIG.  85. 
85  is  used  for  the  process. 


*  Theory  7221  £ 


THALLIUM.  171 

capacity,  containing  platinum- foil  electrodes  of  9  sq.  cm. 
surface,  terminating  in  con  tact- wires  fused  into  the  glass. 
The  thallium  salt  and  about  5  g  ammonium  oxalate  are  dis- 
solved in  this  flask,  and  electrolyzed,  after  dilution,  with  a 
current  of  0.1  ampere.  The  completion  of  the  reaction  is 
ascertained  by  testing  with  ammonium  sulphide.  As  the 


FIG.  86. 

ammonium  oxalate  is  converted  into  carbonate  by  the  current, 
and  the  measuring-tube  would  be  insufficient  to  contain 
the  disengaged  carbon  dioxide,  the  solution  remaining  in  the 
flask  is  removed  after  the  reaction.  This  may  readily  be 
done  by  the  use  of  two  siphons.  Neumann's  automatic 
arrangement  for  this  purpose  is  shown  in  Fig.  86 ;  it  is 
very  convenient  where  many  determinations  are  to  be  per- 


172         QUANTITATIVE   ANALYSIS    BY    ELECTROLYSIS. 

formed,  and  its  operation  is  easily  seen  from  the  figure. 
The  washing  is  conducted  without  interrupting  the  current. 
To  remove  the  gas-bubbles  clinging  to  the  electrode  it  is 
desirable  to  heat  the  flask  a  short  time  after  the  washing  is 
complete.  The  flask  is  then  connected  to  the  measuring- 
tube,  the  thallium  dissolved,  and  the  hydrogen  collected  ani 
measured  in  the  usual  way, 


SILVER. 

LITERATURE  : 

Luckow,  Dingl.  polyt.  Journ.,  178,  43 ;  Zeit.  f.  anal.  Chem.,  19,  15. 

Fresenius  and  Bergmann,  Zeit.  f.  anal.  Chem.,  19,  324, 

Krutwig,  Ber.  deutsch.  chem.  Ges.,  15,  1267. 

Schucht,  Zeit.  f.  anal.  Chem.,  22,  417. 

Einnicutt,  Am.  Chem.  Journ.,  4,  22. 

Riidorff,  Zeit.  f.  angew.  Chem.,  1892,  p.  5. 

Eisenberg,  Dissertation.     Heidelberg,  1895. 

Of  the  methods  proposed  for  the  determination  of  silver, 
the  one  suggested  by  Luckow  (separation  of  the  silver  from 
the  potassium  double  cyanide)  is  probably  the  most  suitable. 
If  insoluble  silver  compounds  (silver  chloride,  silver  oxalate) 
are  to  be  analyzed,  they  are  dissolved  in  potassium  cyanide 
solution.  For  conducting  the  method,  3  g  potassium  cyanide, 
are  added  to  the  solution,  which  is  then  diluted  to  100-120  cc. 
Eisenberg,  who  tested  the  method  in  the  Aachen  laboratory, 
was  convinced  that  the  success  of  the  same,  as  well  as  the 
metallic  condition  of  the  precipitated  silver,  depends  upon  the 
purity  of  the  potassium  cyanide  used.  Even  the  so-called 
"  purissimum  "  potassium  cyanide  of  commerce  is  unsuited. 
It  is  therefore  desirable  to  prepare  pure  potassium  cyanide  by 
passing  hydrocyanic  acid  gas  into  an  alcoholic  solution  of 
potassium  hydroxide. 


SILVER.  173 


CONDITIONS    OF    EXPERIMENT, 

Temperature  of  the  liquid  :   20-30°. 

Current  density:  ND100  —  0.2-0.5  amp. 

Electrode  tension  :   3.7-4.8  volts. 

Time  of  electrolysis :  In  the  presence  of  equal  quantities 
of  silver,  with  current  densities  ND100  =  0.2-0.5  ampere, 
this  varies  from  5  to  1  j-  hours. 

For  this  determination,  roughened  dishes  give  best  results. 

EXPERIMENT. 

(a)  Experiment  with  the  so-called  "  purissimum  "  potas- 
sium cyanide  of  commerce.      Used  silver  sulphate. 

C,,KO*        Current  Electrode  Condition 

Dsc-       Density,  Tension,    Temp.        Time.      Found.*  of  Remark, 

taken.         Amp '      Vokg  Metal 

0.8660     0.4-0.2     3.35     23-24°     6  hr.      68. 91#  }  Not  firmly  j  Rough  dish. 
0.8660      0.3-0.1      3.55      22-24°      6"        68. 91 ")  adherent    (  Polished  " 

(b)  Experiment  with  pure  potassium  cyanide.     Used  silver 
sulphate. 

O,,K-«-         Current     Electrode  Condition 

Dsr-        Density,        Tens.,     Temp.          Time.         Found.  of  Remark, 

taken.          Amp/'        volts!  Metal. 

0.4369  0.3-0.23  3.72  22°        5  hr.  69.08$  )  Firmly     (Roughened 

0.4370  0.52-0.54  4.6-4.8  20-30°  1  "    40  m.  69.00  "J  adherent  1       dish. 

0.4369  0.32-0.21  3.70  22°        5"  69.12")        ((        i    Polished 

0.8742  0.55-0.53  4.0  23°        2  "    40  "    68.98  "  f  \      dish. 

J.  Krutwig  treats  the  solution  of  the  silver  salt  with 
ammonia  in  slight  excess,  adds  ammonium  sulphate,  and 
electrolyzes. 

In  the  Munich  laboratory  the  following  conditions  have 
been  determined  for  the  preceding  process.  The  solution, 
which  must  not  contain  more  than  0.5  g  silver,  is  treated 
*  [Theory  69.22*  Ag.] 


174         QUANTITATIVE  ANALYSIS   BY   ELECTROLYSIS. 

with  20  per  cent  by  volume  of  ammonia  (sp.  gr.  0.96)  and  5$ 
ammonium  sulphate  solution  (1 : 10),  warmed,  and  electrolyzed 
with  a  current  of  ND100  =  0.02  —  0.05  ampere.  After  the 
current  is  stopped  the  precipitate  must  be  very  thoroughly 
washed  to  completely  remove  the  ammonium  sulphate. 

Fresenius  and  Bergmann  have  found  that  silver  can  also 
be  precipitated  in  a  dense  form  from  a  solution  containing 
nitric  acid:  20  cc  of  nitric  acid  (sp.  g.  1.2)  are  added  to  the 
silver  solution,  which  is  then  diluted  with  water  to  about 
200  cc  and  electrolyzed. 

According  to  results  in  the  Munich  laboratory,  it  is  desir- 
able to  add  to  the  solution,  which  may  contain  as  much  as  OA 
g  silver,  3  per  cent  by  volume  of  nitric  acid,  sp.  gr.  1.36,  and 
to  electrolyze  the  heated  solution  with  a  current  of  ND]00  = 
0.04-0.05  ampere.  The  silver  must  be  carefully  washed 
without  interrupting  the  current,  to  prevent  loss.  An  insuf- 
ficient quantity  of  nitric  acid  may  lead  to  the  formation  of 
peroxide. 

MERCURY. 

LITERATURE  : 

Clarke,  Am.  Journ.  of  Sci.  and  Arts,  6,  200. 

Classen  and  Ludwig,  Ber.  deutsch.  chem.  Ges.,  19,  323. 

Hoskinson,  Am.  Chem.  Journ.,  8,  209. 

Smith  and  Knerr,  ibid.,  8,  206. 

Smith  and  Frankel,  Am.  Chem.  Journ.,  11,  264. 

Smith,  Journ.  Anal.  Chem.,  5.  202. 

Vortmann,  Ber.  deutsch.  chem.  Ges.,  24,  2749. 

Brandt,  Zeit.  f.  angew.  Chem.,  1891,  p.  202. 

Riidorff,  ibid.,  1892,  p.  5. 

Eisenberg,  Dissertation.     Heidelberg,  1895. 

Frankel,  Journ.  Franklin  Inst.,  131,  144. 

Rising  and  Lenher,  Berg*  und  Hiittenm.  Ztg.,  55,  175. 

The  metal  can  be  readily  separated  from  solutions  of  the 
mercuric  salts  to  which  4-5  g  ammonium  oxalate  have  been 


MERCURY. 


175 


added  (method  of  the  author).  If  the  mercury  is  present  as 
chloride  in  the  solution,  the  electrolysis  is  continued  until 
mercurous  chloride  disappears  from  the  positive  electrode^ 

CONDITIONS    OF    EXPERIMENT. 

Temperature  of  liquid  :   Ordinary  temperatures. 

Current  density:  NT)100  =  0.1-1.0  amp. 

Electrode  tension:   2.5-5.5  volts. 

Time :   Dependent  on  the  current  density. 

Roughened  dishes  are  preferable  to  polished,  on  account 
of  the  more  uniform  distribution  and  firmer  adherence  of  the 
mercury  to  the  cathode.  On  polished  dishes  the  mercury 
separates  in  the  form  of  small  globules. 


EXPERIMENT. 


Subst. 
used. 
HgCla. 

g- 

Current 
Density, 
Amperes. 

Electrode 
Tension, 

Voles. 

Temp. 

Time, 
hrs.  m. 

Found'*    Remark. 

0.4068 

0.2  -0.15 

2.6  -3.35 

30-23° 

5 

15 

73.74) 

0.4073 

1.02-0.93 

4.05-4.75 

29-37° 

1 

30 

73.63 

0.4076 

1.08-0.92 

4.42-4.88 

25-40° 

2 

5 

73.77 

Roughened 

0.4080 

1.15-1.09 

4.97-5.05 

18-40.5° 

2 

5 

73.87 

Dish. 

0.4080 

1.12-0.93 

4.95-4.85 

18-38° 

2 

5 

73.84 

0.4080 

1.52-0.48 

3.65-4.65 

16-27° 

3 

55 

73.67-1 

0.4070 

0.2  -0.23 

2.89-3.75 

28^24° 

5 

15 

73.80 

0.4073 

1.06-0.95 

4.45-5.00 

30-39.5° 

1 

30 

73.29 

0.4076 

1.16-1.09 

5.32-5.53 

23-40° 

2 

5 

73.93 

Polished 

0.4080 

1.20-0.99 

4.70-4.90 

18-43° 

3 

73.55 

Dish. 

0.4075 

1.51-0.48 

3.87-4.50 

16-30° 

3 

55 

73.85 

Mercury  may  also  be  quantitatively  precipitated  from  a 
solution  containing  nitric,  sulphuric,  or  hydrochloric  acid. 
If  no  other  metal  than  mercury  is  present,  1-2  per  cent  by 
volume  of  nitric  acid  is  sufficient ;  while  in  the  presence  of 
other  metals,  which  are  not  precipitated  from  solutions  con- 

*  [Theory  73.85#  Hg.] 


176         QUANTITATIVE   ANALYSIS   BY    ELECTROLYSIS. 

taining  free  acid,  5  per  cent  by  volume  is  required.  In  the 
latter  case  a  current  density  not  greater  than  0.5  ampere  is 
employed ;  in  the  former  the  current  density  may  be  raised 
to  1  ampere. 

If  hydrochloric  acid  is  used,  only  a  few  drops  are  added, 
since  larger  quantities  have  a  detrimental  action  on  the  separa- 
tion of  the  metal.  Large  quantities  of  chlorides  have  an 
action  similar  to  that  of  large  quantities  of  hydrochloric  acid. 

Insoluble  mercury  compounds  may  be  easily  electrolyzed 
by  suspending  them  in  water  slightly  acidified  with  hydro- 
chloric acid,  or  in  a  dilute  solution  of  sodium  chloride  (about 
10  per  cent).  This  process,  originated  by  the  author,  is 
used  at  Almaden  for  determining  the  amount  of  mercury 
contained  in  cinnabar. 

Edgar  F.  Smith  precipitates  mercury  from  the  solution  of 
the  same  in  potassium  cyanide.  The  solution  of  the  mercuric 
salt,  which  may  contain  about  0.2  g  mercury,  is  decomposed 
with  0.25-2  g  potassium  cyanide,  diluted  with  water  to  175 
cc  and  electrolyzed. 

Heidenreich,  in  the  Aachen  laboratory,  determined  the 
conditions  of  experiment  for  this  method. 

CONDITIONS    OF    EXPERIMENT. 

Temperature  of  the  liquid :  Ordinary  temperatures. 
Current  density :  KD100  =  0.03-0.08. 
Electrode  tension :   1 . 6  5-1 .  T  5  volt. 
Time :  Dependent  on  the  current  density. 

m  ,  Current  Dens.     Tension,         m._,  •&       j  * 

Taken-  ND100,  Amp.          Volts.  Time"        Found.* 

0.2501  g  HgCU,  2-3  g   KCN     0.08-0.04     1.65-1.69       5  hr.    73.61$  Hg 
0.2655  "       "         "    "       "  0.03  1.75          14  "      73.50"  " 

The  metal  reduced  by  this  method  must  be  washed  with 
water   only,   and    not  with  alcohol,   since,  on  washing  with 
*  [Theory  73.85$  Hg.] 


GOLD.  177 

the  latter,  small  quantities  of  the  mercury  will  become  loos- 
ened and  be  carried  away. 

GOLD. 

LITERATURE  I 

Luckow,  Zeit.  f.  anal.  Chem.,  19,  14. 

Brugnatelli,  Phil.  Magazin,  21,  187. 

Smith  and  Muhr,  Ber.  deutsch.  chem.  Ges.,  23,  2175. 

Smith  and  "Wallace,  Proceed.  Chem.  Soc.  Franklin  Inst.,  3,  20. 

Smith,  Am.  Chem.  Journ.,  13,  206. 

Persoz,  Annal.  Chem.  Pharm.,  65,  164. 

Riidorff,  Zeit.  f.  angew.  Chem.,  1892,  p.  695. 

Gold  may  be  separated  in  a  compact  form  from  solutions 
of  gold  salts  in  potassium  cyanide.  To  form  the  double 
cyanide,  about  3  g  of  potassium  cyanide  are  added.  The 
solution  is  then  electrolyzed  at  ordinary  temperatures  or  at 
temperatures  between  50°  and  60°.  Since  the  gold  can  be 
removed  from  the  platinum  dish  with  aqua  regia  only  (an 
operation  by  which  the  platinum  is  also  dissolved),  platinum 
dishes  coated  with  a  thin  deposit  of  silver  have  previously  been 
used  for  this  determination.  According  to  a  private  commu- 
nication from  Dr.  "W.  Dupre  of  Stassfurt,  the  gold  may  be 
readily  removed  from  the  platinum  dishes  by  warming  with  a 
solution  of  chromic  anhydride  in  saturated  sodium  chloride 
solution.  The  author  can  confirm  this  statement;  in  this 
operation  gold  only,  and  no  platinum,  goes  into  solution. 

Since  the  conditions  of  experiment  for  the  separation  of 
gold  from  double  cyanides  had  not  been  previously  determined, 
they  were  ascertained  by  Dr.  v.  Wirkner  at  the  suggestion  of 
the  author. 

CONDITIONS    OF    EXPERIMENT. 

Temperature  of  the  liquid:  Ordinary  temperatures;  or 
better  50-60°,  since  at  ordinary  temperatures  a  brownish 
decomposition  product  of  potassium  cyanide  often  separates. 


178        QUANTITATIVE  ANALYSIS   BY   ELECTROLYSIS. 

Current  density  :  ND100  =  0.3-0.8  amp. 
Electrode  tension :  2.7-4  volts. 


EXPERIMENT. 


A  solution  of  chloride  of  gold  of  unknown  strength  was 
used.  The  electrolyses  were  carried  out  in  roughened  plat- 
inum-iridium  dishes  without  a  coating  of  silver.  Used  3  g 
potassium  cyanide,  120  cc  liquid. 


Taken,  cc 
Gold  Chlo- 
ride Sol. 

Current 
Density, 
Amperes. 

Electrode 
Tension, 
Volts. 

Temper- 
ature. 

Time, 
hr.     m. 

15 

0.3 

3.5-3.9 

20-27° 

5    — 

15 

0.35 

3.9-4.0 

22-28° 

14  (overnight) 

30 

0.37 

3.6-3.9 

20-28° 

4    15 

15 

0.38 

2.7-3.8 

52-55° 

1     30 

15 

0.38 

2.7-3.4 

53-54° 

1     20 

15 

0.39 

2.7-3.8 

52-56° 

1    30 

15 

0.85 

4.0-4.1 

52-56° 

1    30 

Found, 
g- 

0.0545 
0.0548 
0.1099 
0.0544 
0.0546 
0.0545 
0.0544 


ANTIMONY. 

LITERATURE  I 

Wrightson,  Zeit.  f.  anal.  Chem.,  15,  300. 

Parodi  and  Mascazzini,  ibid.,  18,  588. 

Luckow,  ibid,,  19,  13.  » 

Classen  and  v.  Reiss,  Ber.  deutsch.  chem.  Ges.,  14,  1622  ; 

ibid.,  17,  2467;  18,  1104. 
Lecrenier,  Chemiker  Zeitung,  13,  1219. 
Vortmann,  Ber.  deutsch.  chem.  Ges.,  24,  2762. 
Rudorff,  Zeit.  f.  angew.  Chem.,  1892,  p.  199. 
Classen,  Ber.  deutsch.  chem.  Ges.,  27,  2060. 

Antimony  is  precipitated  from  hydrochloric  acid  solution, 
but  not  in  an  adherent  form.  If  potassium  oxalate  is  added 
to  the  solution  of  the  trichloride,  antimony  is  easily  reduced, 
but  adheres  even  less  closely  than  in  the  other  case.  An 
adherent  metallic  deposit  can  be  obtained  by  adding  potassium 
tartrate,  but  the  separation  is  then  too  slow. 


ANTIMONY.  179 

The  precipitation  of  antimony  from  the  solutions  of  its 
sulpho-salts  is  complete  and  satisfactory.  If  ammonium  sul- 
phide is  used  to  produce  a  double  salt,  it  must  contain  neither 
free  ammonia  nor  polysulphides.  Ammonium  hydrosulphide, 
therefore,  is  convenient  for  the  determination  ;  it  is  kept  in 
small,  tightly  corked  bottles. 

When  a  solution  of  antimony  containing  ammonium  sul- 
phide is  electrolyzed,  there  is  formed  over  the  metal  a  coating 
of  sulphur  which  cannot  be  washed  off  with  water.  When 
the  metal  is  washed  afterward  with  alcohol,  the  thin  coating 
of  sulphur  can  be  removed  by  rubbing  with  the  finger  or  a 
handkerchief  moistened  with  alcohol,  without  danger  of  loss. 

The  use  of  ammonium  sulphide  has  the  disadvantage  that, 
when  several  determinations  are  made  together,  the  odor 
becomes  unbearable.  For  this  reason  the  author  has  made  a 
series  of  experiments  with  potassium  and  sodium  monosul- 
phide  and  hydrosulphide,  the  results  of  which  show  that  the 
precipitation  of  antimony  from  double  salts  with  these  com- 
pounds proceeds  satisfactorily.  As  sodium  sulphide  (Na2S)  is 
the  one  of  the  salts  named  which  is  most  desirable  for  facilitat- 
ing the  separation  of  antimony  from  tin  and  arsenic,  the 
following  particulars  relate  exclusively  to  the  use  of  this  salt  * 
for  the  determination  of  antimony. 

The  following  equations  probably  represent  the  reactions 
which  take  place  in  the  electrolysis  of  the  antimony  sulpho- 
salt.  The  current  first  decomposes  water  : 

3HaO  =  6H  +  30. 
At  the  cathode  : 

SbaS,  +  3NaaS  +6H  =  2Sb  +  6NaHS. 
At  the  anode  : 

3O  =  3Na3Sa  +  3H2O. 


*  For  the  preparation  of  this  salt,  see  section  on  Reagents. 


180         QUANTITATIVE  ANALYSIS   BY    ELECTROLYSIS. 

The  reduction  of  antimony  from  the  prepared  sulpho-salt& 
can  be  carried  out  as  well  at  ordinary  as  at  higher  tempera- 
tures. In  the  first  case  the  determination  requires  17-18 
hours,  in  the  latter  about  2  hours.  If  the  separation  is  con- 
ducted in  polished  dishes,  only  relatively  small  quantities  of  the 
metal  can  be  made  to  adhere  firmly  to  the  dish,  and  the  em- 
ployment of  weak  currents  is  necessary.  In  recent  experi- 
ments roughened  dishes  have  been  used,  and  the  reduction  has 
been  conducted  from  hot  solutions  and  with  stronger  currents. 

To  carry  out  the  analysis,  80-100  cc  of  a  solution  of 
sodium  monosulphide  (sp.  g.  about  1.14)  are  added  to  the 
antimony  solution,  which  is  diluted  with  water  to  120  cc,  and 
electrolyzed.  If  the  metal  is  precipitated  from  a  warm  solu- 
tion, it  must  be  washed  without  interrupting  the  current. 
The  end  of  the  reaction  can  only  be  determined  with  certainty 
by  the  use  of  another  electrode  which  is  dipped  into  the  liquid 
and  brought  into  contact  with  the  dish,  i.e.,  the  cathode. 

The  dish  with  the  separated  antimony  is  treated  in  the 
usual  way  with  water  and  perfectly  pure  absolute  alcohol, 
dried  for  a  short  time  in  the  air-bach  at  80-90°,  and  weighed. 

CONDITIONS    OF    EXPERIMENT. 

A.   Temperature  of  the  liquid  :  Ordinary  temperatures. 
Current  density:   ND100  =  0.3-0.35  ampere. 
Electrode  tension:   1-1.8  volts. 
Remark :  It  is  best  to  use  roughened  dishes  only. 

EXPERIMENT. 

Used  tartar  emetic.* 


Subst. 
taken, 

Current 
Density, 

Electrode 
Tension, 

Time. 

Found, 

Condition 
of  the 

g- 

Amp. 

Volts. 

%' 

Metal. 

0.7892 
0.7894 

0.35 
0.35 

1.70-1.06 
1.80-1.00 

17  hr.  30  m. 

17  "    30  " 

37.84 
37.80 

(  bright     rae- 
•<  tallic,  adher- 
(  ent. 

*  [Probably  impure  anhydrous,  KSbC4H4O7  containing  about  37.12$ 
Sb.— Trans.] 


ANTIMONY. 


181 


B.   Temperature  of  the  liquid :   55-70°. 
Current  density:  KD100  =  1.0-1.5  amp. 
Electrode  tension  :   1-2  volts. 


Subst. 
taken, 

Current 
Density, 

Electrode 
Tension, 

Temp. 

Time. 

Found, 

Condition 
of  the 

g.            Amp. 

Volts. 

hr. 

m. 

% 

Metal. 

0.7895 

1. 

00-1, 

2 

1 

.45-1.25 

55-60° 

2 

5 

37.64 

( 

0.7895 

1.06-1, 

25 

1 

.35 

65-70° 

2 

15 

37.57 

1 

bright 

0.7898 

1. 

50 

1 

.42 

70° 

1 

45 

37.85 

J\ 

metallic 

1.5873 

1. 

50 

1 

.80 

70-80° 

2 

30 

37.84 

The  method  of  determining  antimony  in  solutions  of  the 
polysulphides  of  the  alkalies  is  very  simple.  The  solution 
containing  polysulphides  is  treated  with  an  excess  of  hydrogen 
peroxide  and  heated  till  it  becomes  colorless.  If  a  great 
excess  of  hydrogen  peroxide  is  used,  it  may  happen  that  the 
alkali  sulphide  is  entirely  decomposed  and  antimony  sulphide 
precipitated.  If  the  solution  is  entirely  colorless,  or  if  a  pre- 
cipitate of  antimony  sulphide  has  already  appeared,  the  solu- 
tion is  cooled,  80  cc  of  a  solution  of  sodium  monosulphide 
are  added,  the  whole  is  diluted  with  water  to  about  120-150 
cc,  and  electrolyzed  as  above  directed. 

[Chittenden  and  Blake  *  have  applied  the  electrolytic 
method  to  the  determination  of  very  small  quantities  of  anti- 
mony in  a  large  amount  of  organic  matter.  In  test  experi- 
ments, 100  g  of  beef  or  liver  were  finely  divided,  a  few  cubic 
centimeters  of  a  standard  antimony  solution  added,  the  mixture 
thoroughly  oxidized  with  hydrochloric  acid  and  potassium 
chlorate,  all  free  chlorine  removed  by  heat,  and  the  antimony 
precipitated  by  hydrogen  sulphide.  The  precipitate  containing, 
together  with  antimony  sulphide,  some  sulphur  and  organic 
matter,  was  dissolved  in  cold  sodium  monosulphide,  and  directly 
submitted  to  the  action  of  a  current  from  four  gravity  cells  of 


*  Trans.  Conn.  Acad.  Arts  and  Sci.,  7,  276. 


182         QUANTITATIVE  ANALYSIS   BY   ELECTROLYSIS. 

moderate  size.  The  electrolytic  action  was  continued  till  all 
the  organic  matter  and  sulphur  was  oxidized  (eighteen  to  forty- 
eight  hours),  and  the  deposited  antimony  washed  without 
breaking  the  current.  Results  satisfactory,  and  much  better 
than  those  obtained  by  any  other  process. 

Chittenden  and  Blake  also  found  that  antimony  in  small 
quantities  was  deposited  quantitatively  from  urine  by  acidify- 
ing with  sulphuric  acid  (1  cc  dilute  H2SO4  to  25  cc  urine), 
and  submitting  directly  to  electrolysis.  The  battery  used 
was  the  same  as  before. — Trans. ] 

PLATINUM. 

LITERATURE  : 

Luckow,  Zeit.  f.  anal.  Chem.,  19,  13. 
Classen,  Ber.  deutsch.  cheni.  Ges.,  17,  2467. 
Smith,  Am.  Chem.  Journ.,  13,  206. 
Eudorff,  Zeit.  f.  angew.  Chem.,  1892,  p.  696. 

The  compounds  of  platinum  are  very  readily  decomposed 
by  the  electric  current,  the  metal  being  precipitated.  Accord- 
ing to  the  determinations  made  by  Dr.  W.  Gobbels  in  the 
Aachen  laboratory,  if  a  solution  of  a  platinum  salt  containing 
2-3  per  cent  by  volume  of  sulphuric  acid  is  used  for  the  de- 
composition and  is  electrolyzed  with  a  current  of  N"D]00  =  0.1- 
0.2  ampere,  all  the  platinum  separates  in  a  short  time  in  the 
form  of  platinum-black.  If,  however,  a  solution  heated  to 
60-65°  is  electrolyzed  with  a  current  of  ND100  =  0.05  amp. 
and  1.2  volts  tension,  the  platinum  separates  quantitatively 
and  in  a  very  compact  form.  The  reduced  metal  is  so  dense 
that  it  cannot  be  distinguished  from  hammered  platinum. 

If  the  quantity  of  platinum  is  about  0.4  g,  the  solution  of 
the  platinum  salt,  according  to  the  practice  in  the  Munich 
laboratory,  is  treated  with  2  per  cent  by  volume  dilute  sul- 


PALLADIUM — TIN.  183 

phuric  acid  (1:5),  heated,  and  electrolyzed  with  a  current  of 
ND100  =  0.01  —  0.03  ampere;  the  precipitation  is  complete 
in  about  5  hours. 

Iridium  is  not  reduced  from  its  solutions  by  a  current  of 
ND100  =  0.05  amp.  and  1.2  volts  tension:  this  property  may 
be  used  for  the  quantitative  separation  of  platinum, from  irid- 
ium  (Classen). 

PALLADIUM, 

LITER  AT  CTRE  I 

Wohler,  Lieb.  Ann.,  133,  357. 
Schucht,  Zeit.  f.  anal.  Chem.,  22,  242. 
Smith  and  Knerr,  Am.  Chem.  Journ.,  12,  252. 
Smith,  ibid.,  8,  206  ;  14,  435. 

Palladium  is  determined  in  the  same  way  as  platinum. 
If  a  current  of  ND100  =  0.05  ampere,  with  a  tension  of  1.2 
volts,  is  used  for  the  reduction,  the  palladium  is  obtained  in 
an  excellent  metallic  condition. 

TIN. 

LITERATURE  I 

Luckow,  Zeit.  f.  anal.  Chem.,  19,  13. 

Classen  and  v.  Reiss,  Ber.  deutsch.  chem.  Ges.,  14,  1622. 

Gibbs,  Chem.  News,  42,  291. 

Classen,  Ber.  deutsch.  chem.  Ges.,  17,  2467  ;  18,  1104. 

Bongartz  and  Classen,  ibid.,  21,  2900. 

Rudorff,  Zeit.  f.  angew.  Chem.,  1892,  p.  196. 

Classen,  Ber.  deutsch.  chem.  Ges.,  27,  2060. 

Engels,  Zeit.  f.  Elektrochemie,  1895-96,  p.  418. 

Freudenberg,  Zeit.  f.  phys.  Chem.,  12,  121. 

Heideureich,  Ber.  deutsch.  chem.  Ges.,  28,  1586. 

Tin  separates  completely  from  a  solution  containing  the 
ammonium  double  oxalate,  or  from  an  ammonium  sulphide 
solution.  Sodium  and  potassium  sulphides  cannot  be  used, 


184        QUANTITATIVE   ANALYSIS   BY   ELECTROLYSIS. 

as  tin  separates  only  partially  from  a  dilute  solution  of  the 
corresponding  sulpho-salt,  and  not  at  all  from  a  concentrated 
solution. 

If  tin  is  precipitated  from  the  ammonium  double  oxalate, 
a  separation  of  stannic  acid  readily  occurs,  especially  when 
much  tin  is  present,  which  must  be  redissolved  by  addition 
of  oxalic  acid.  •  The  reduction  of  tin  may  be  carried  out 
without  difficulty,  however,  if  acid  ammonium  oxalate  is  used 
instead  of  the  neutral  oxalate.  The  results  obtained  by  this 
process  are  so  accurate  that  the  author  has  found  it  adapted 
to  the  determination  of  the  atomic  weight  of  tin.* 

The  solution  of  tin  is  treated  with  a  cold  saturated  solu- 
tion of  acid  ammonium  oxalate  in  the  proportion  of  20  cc  to 
0.1  g  tin.  The  solution  is  diluted  to  about  150  cc  and  elec- 
trolyzed.  The  tin  is  completely  precipitated  as  a  closely  ad- 
herent, shining,  silver- white  metal,  even  when  as  much  as  6  g 
is  present.  The  current  is  interrupted,  and  the  metal  washed 
as  usual  with  water  and  alcohol,  and  dried  at  80°-90°. 

CONDITIONS    OF    EXPERIMENT. 

Temperature  of  liquid  :   Ordinary  temperatures. 
Current  density  :  ND100  =  0.2-0.6  amp. 
Electrode  tension:   2.7-3.8  volts. 
Time  of  experiment :    8-10  hours. 

EXPERIMENT. 

Used  0.9-1  g  SnCl4.2NH4Cl[32.10#  Sn],  120  cc  of  a  satu- 
rated solution  of  acid  ammonium  oxalate. 


Current  Density, 
Amperes. 

Electrode  Tension, 
Volts. 

Temp. 

Time. 

Found. 

02-0.3f 

2.7-3.8 

25° 

8  hr.  5.  m. 

32.062 

0.3-0.6 

2.8-3.8 

30-35° 

9  "  45  " 

32.00" 

*  Bongartz  and  Classen,  Ber.  deutch.  chem.  Ges.,  21,  2900. 
f  Finally  increased  to  0.5  ampere. 


TIN.  185 

If  larger  quantities  of  the  tin  salt  are  used,  it  is  necessary  to 
add  acid  ammonium  oxalate  from  time  to  time,  on  account  of 
the  decomposition  of  the  acid  ammonium  oxalate,  which 
causes  the  solution  to  react  alkaline.  According  to  recent 
investigations,  the  determination  of  tin  may  be  carried  out 
by  treating  the  solution  of  the  tin  salt  with  neutral  ammonium 
oxalate  to  form  the  double  salt,  acidifying  .with  oxalic  acid 
and  electrolyzing  warm. 

Heidenreich,  who  tested  this  method  in  the  Aachen 
laboratory,  found  that  the  determination  of  tin  can  be  com- 
pleted in  4-4^  hours.  4  g  ammonium  oxalate  to  every 
0.3  g  tin  present  are  added  to  the  solution,  which  is  then 
acidified  with  9-10  g  oxalic  acid,  warmed  to  60-65°,  and 
electrolyzed  with  a  current  of  ND100=  1-1.5  amperes.  The 
precipitate  must  be  washed  without  interrupting  the  current. 

Instead  of  oxalic  acid,  acetic  acid  may  be  used ;  it  possesses, 
however,  no  advantages.  100  cc  of  a  saturated  solution  of 
ammonium  oxalate  are  added  to  the  solution  of  the  tin  salt, 
which  is  then  acidified  with  25  cc  acetic  acid  (sp.  g.  1.0615; 
about  50$).  The  metal  is  precipitated  in  the  form  of  radi- 
ated crystals,  in  contrast  to  the  precipitate  from  acid  ammo- 
nium oxalate  solutions.  Tin  adheres  better  to  roughened 
than  to  polished  dishes. 

The  following  experiments  were  conducted  by  the  acetic 
acid  method : 

Current  Density,  Electrode       Tom™  Timo  i?rmn,i 

Ampere.  Tens.,  Volts.        3mp' 

0.3  increased  to  0.5        3.2-3.8        25°  6  hr.  15  m.       32.00$ 

0.5        "          "  1.0        3.6-4.2        25-30°        5  "     45  "         32.01" 

In  these  experiments  the  tin  in  the  polished  dishes  ap- 
peared brilliantly  crystalline,  and  in  the  roughened  dishes 
silver- white. 

Since  tin,  like  zinc,  is  dissolved  with  difficulty  from  the 


186        QUANTITATIVE  ANALYSIS   BY   ELECTROLYSIS. 

platinum  dishes  by  acids,  it  is  necessary  to  use  fused  acid 
potassium  sulphate  to  remove  it.  It  is  therefore  best  to  pre- 
cipitate the  tin  in  coppered  dishes  (see  Zinc). 

Engels  worked  out  the  following  method  in  the  Aachen 
laboratory :  The  tin  salt  is  dissolved  in  water  containing  a 
few  cubic  centimeters  of  oxalic  acid,  and  0.3-0. 5  g  hydroxyl- 
amine,  2  g  ammonium  acetate,  and  2  g  tartaric  acid  are  added 
for  every  0.5-1.2  g  tin  salt  taken.  The  solution  is  then 
diluted  to  150  cc. 

CONDITIONS    OF    EXPERIMENT. 

Temperature  of  liquid :  60-70°. 
Current  density:  ND100  =  0.99-1  amp. 
Electrode  tension :  4-5  volts. 
Time :   3-5  hours. 

EXPERIMENT. 


SnCl4. 
2NH401, 
g- 

0.9175 

Current 
Density, 
Amp. 

1-0.8 

Electrode 
Tension, 
Volts. 

5.2-5.6 

Temp. 
70° 

Time. 
3hr. 

g.F°  Percent.  Calculated. 
0.2970      32.37      32.37# 

0.9859 

1-0.8 

4.8-5.3 

63° 

3  " 

0.3195 

32.40 

0.9050 

1-0.9 

5.0-5.6 

65° 

3  " 

0.2931 

32.39 

1.1879 

0.5 

5.1-6.0 

45° 

6  " 

0.3847 

32.38 

1.0026 

0.7 

3.4 

60° 

3  " 

0.3238 

32.36 

0.9940 

0.7 

4.0 

60° 

3£" 

0.3219 

32.38 

1.0024 

0.8 

4.6 

60° 

3  " 

0.3250 

32.42 

1.0022 

0.8 

4.2-4.4 

60° 

3  " 

0.3252 

32.44 

In  the  solution  of  the  ammonium  sulpho-salt  tin  behaves 
like  antimony.  The  tin  solution  (if  necessary  after  neutraliza- 
tion with  ammonia)  is  treated  with  ammonium  sulphide  free 
from  ammonia  (no  more  is  added  than  is  needed  to  form  the 
sulpho-salt),  diluted  to  150-175  cc,  warmed  to  50-60°,  and 
electrolyzed  with  a  current  of  1-2  amperes,  at  a  tension  of  3. 5- 
4  volts.  Under  these  conditions  0.3-0.4  g  of  tin  can  be 
reduced  in  an  hour.  Sometimes  a  deposit  of  sulphur  adheres 
so  strongly  to  the  tin  at  the  edge  of  the  dish  that  it  cannot  be 


TIN.  187 

washed  off  with  water ;  it  may,  however,  be  easily  removed, 
after  washing  with  alcohol,  by  gentle  rubbing  with  a  linen 
cloth. 

In  gravimetric  analysis  tin  is  often  separated  from  other 
metals  by  sodium  sulphide  instead  of  ammonium  sulphide. 
In  order  to  determine  the  tin  electrolytically  in  such  cases, 
the  sodium  sulphide  must  be  converted  into  ammonium  sul- 
phide.* To  accomplish  this,  the  solution  is  treated  with 
about  25  g  pure  ammonium  sulphate  free  from  iron,  and 
heated  very  carefully,  with  the  dish  covered,  till  the  hydrogen 
sulphide  has  all  escaped ;  the  solution  is  then  kept  in  gentle 
ebullition  for  about  fifteen  minutes.  Complete  conversion 
into  ammonium  sulphide  is  shown  by  the  greenish-yellow 
color  of  the  solution.  If  the  heating  is  continued  too  long, 
tin  sulphide  may  separate ;  it  can  be  dissolved  in  ammonium 
sulphide.  After  it  is  completely  cooled,  any  sodium  sulphate 
that  may  have  separated  is  dissolved  by  addition  of  water, 
and  the  solution  electrolyzed. 

The  determination  of  the  tin  is  much  more  simply  and 
easily  accomplished  by  converting  the  solution  of  tin  sulphide 
in  sodium  sulphide  into  the  acid  oxalate.  This  conversion 
may  be  accomplished  in  two  ways ;  either  the  sulpho-salt  is 
decomposed  with  dilute  sulphuric  acid  to  remove  the  greater 
part  of  the  sulphur  as  hydrogen  sulphide,  and  the  separated 
tin  sulphide  oxidized  with  hydrogen  peroxide  f  until  the  stan- 
nic acid  which  is  produced  appears  clear  white,  or  the  heated 
alkaline  solution  is  treated  directly  with  hydrogen  peroxide 
(of  which  a  great  quantity  is  needed),  then  acidified  with  sul- 
phuric acid  to  precipitate  stannic  acid,  neutralized  with 

*  Sodium  sulphide  cannot  be  replaced  by  potassium  sulphide  in  the 
separation  from  other  metals,  because  the  latter  produces  difficultly  soluble 
potassium  sulphate  when  ammonium  sulphide  is  formed. 

f  Classen  and  Bauer,  Ber.  d.  ch.  Ges.,  16,  1062. 


188         QUANTITATIVE   ANALYSIS   BY   ELECTROLYSIS. 

ammonia,  and  treated  with  more  hydrogen  peroxide.  In 
either  case  the  solution  is  heated  to  decompose  the  excess  of 
hydrogen  peroxide,  and  the  stannic  acid  allowed  to  settle  and 
then  filtered  off.  The  precipitate  is  washed  with  the  oxalate 
solution  from  the  filter  into  a  beaker,  the  filter  washed  with 
hot  oxalic  acid  solution,  and  the  stannic  acid  in  the  beaker 
dissolved  by  heating.  Sometimes  there  is  a  residue  of  sulphur, 
which  is  removed  by  filtration.  The  filtrate  is  collected  in 
the  weighed  platinum  dish  to  be  used  for  the  electrolysis,  and 
the  sulphur  is  washed  with  a  cold  saturated  solution  of  am- 
monium oxalate  or  acid  ammonium  oxalate.  The  solution 
for  electrolysis  must  contain  at  least  4  g  of  the  oxalate. 

ARSENIC. 

LITERATURE  I 

Luckow,  Zeit.  f.  anal.  Ohem.,  19,  14. 

Classen  and  v.  Reiss,  Ber.  deutsch.  chem.  Ges.,  14,  1622. 

Moore,  Chem.  News,  53,  209. 

Vortmann,  Ber.  deutsch.  chem.  Ges.,  24,  2764. 

Arsenic  cannot  be  quantitatively  separated  either  from 
aqueous  solutions  or  from  solutions  containing  hydrochloric 
acid,  ammonium  oxalate,  or  alkali  sulphides.  From  aqueous 
as  from  oxalic  acid  solutions  a  part  of  the  metal  is  reduced, 
while  from  hydrochloric  acid  solutions,  if  the  current  is  al- 
lowed to  act  for  a  sufficient  length  of  time,  all  of  the  arsenic 
passes  off  in  the  form  of  arseniuretted  hydrogen. 

The  behavior  of  arsenic  (present  as  arsenic  acid)  in  a  con- 
centrated solution  of  sodium  sulphide  permits  the  separation 
of  arsenic  from  antimony,  as  will  be  shown  later. 

POTASSIUM,   AMMONIUM.    (NITROGEN.) 

Potassium  and  ammonium  may  be  determined,  as  is  well 
known,  by  converting  them  into  potassium  or  ammonium  pla- 
tinchloride,  and  weighing  the  precipitate,  dried  at  110°,  on  a 
tared  filter.  This  method,  which  is  almost  universally  em- 


DETERMINATION   OF   NITKIC   ACID   IN   NITEATES.      189 

ployed  in  the  separation  of  potassium  from  sodium,  has  many 
disadvantages.  It  is  preferable,  after  precipitating  and  wash- 
ing the  platinum  salt  as  usual,  to  dissolve  it  in  water,  and 
determine  the  platinum  as  directed  on  p.  182. 

DETERMINATION   OF   NITRIC    ACID   IN   NITRATES. 

As  is  well  known,  nitric  acid  is  often  converted  into  am- 
monia, and  the  latter  determined.  The  action  of  the  galvanic 
current  converts  nitric  acid  into  ammonia,  as  explained  in  the 
Introduction  (p.  3).  If  the  solution  of  an  alkali  nitrate,  acid- 
ified with  dilute  sulphuric  acid,  is  exposed  to  the  action  of  the 
galvanic  current,  no  ammonia  is  formed. 

Luckow  discovered  that  reduction  of  the  nitric  acid  always 
takes  place  when  a  salt  from  which  the  metal  is  precipitated 
by  the  current  is  also  present  in  the  solution.  Copper  salts 
are  best  adapted  for  this  purpose.  G.  Vortmann  has  deter- 
mined in  the  Aachen  laboratory  the  conditions  for  the  quan- 
titative determination  of  nitric  acid  in  nitrates.  The  solution 
of  the  nitrate  is  treated  with  a  sufficient  quantity  of  copper 
sulphate  (in  the  analysis  of  potassium  nitrate,  e.g.,  half  as 
much  crystallized  copper  sulphate  as  potassium  nitrate),  acidi- 
fied with  dilute  sulphuric  acid,  and  electrolyzed  cold.  When 
the  reaction  is  complete  the  solution  is  poured  off,  sodium  hy- 
droxide solution  is  added,  and  the  ammonia  distilled  off  and 
determined  volumetrically  in  the  usual  way.  For  this  pur- 
pose one-fifth  normal  solutions  of  ammonia  and  sulphuric 
acid -are  used.  To  standardize  the  sulphuric  acid,  a  weighed 
quantity  (0.5  g)  of  crystallized  copper  sulphate  is  decom- 
posed electrolytically,  and  the  resulting  free  acid  tit- 
rated with  ammonia.  G.  Vortmann  decomposed  0.4876  g 
CuSO4. 5H2O,  and  used,  for  the  neutralization  of  the  acid  set 
free,  19.6  cc  of  ammonia  of  a  strength  equal  to  the  one- 
fifth  normal  sulphuric  acid.  1  cc  of  the  latter  corresponds 
therefore  to  0. 0028017  g  of  nitrogen  in  the  form  of  ammonia. 


190         QUANTITATIVE  ANALYSIS   BY   ELECTROLYSIS. 


DETERMINATION  OF  THE  HALOGENS. 

Chlorine,  Bromine,  Iodine. 
LITERATURE : 

Vortmann,  Monatshefte  f.  Chem.,  15,  280;  16,  674 ; 
Elektrochem.  Zeit.,  1894  (1),  p.  137. 

The  method  originated  by  Yortmann  depends  upon  the 
principle  that  the  halogens  are  set  free  from  solutions  of 
halogen  salts  by  the  electric  current,  and  while  in  the  ion 
state  combine  with  a  silver  anode  to  form  insoluble  silver 
halide.  The  increase  in  weight  of  the  anode  gives  directly 
the  quantity  of  halogen  which  has  separated.  The  comple- 
tion of  the  analysis  is  determined  by  replacing  the  original 
silver  anode  by  a  second  weighed  silver  anode  and  noting  its 
increase  in  weight. 

For  an  experimental  test  of  the  method,  a  weighed  quantity 
of  iodide  is  dissolved  in  water,  6-10  cc  of  a  10$  solution  of 
sodium  hydroxide  added,  and  the  solution  diluted  to  100-150 
cc.  The  silver  anode,  having  the  form  of  a  watch-glass  6  cm. 
in  diameter,  is  fixed  about  5  mm.  from  the  bottom  of  the  cop- 
per dish  which  serves  as  cathode. 

The  cold  solution  is  electrolyzed  with  a  current  strength 
of  0.03-0.07  ampere  and  a  tension  of  2  volts.  After  4-5 
hours  the  greater  part  of  the  iodine  has  been  converted  into 
silver  iodide,  and  the  remainder  may  be  separated  on  a  fresh 
silver  anode,  after  the  addition  to  the  solution  of  sodium  po- 
tassium tartrate.  The  liquid  is  warmed  to  50-70°  and  elec- 
trolyzed with  a  current  having  a  tension  of  1.2-1.3  volts  and 
a  current  strength  of  0.01-0.02  ampere. 


SEPARATION   OF   METALS.  391 


SEPARATION  OF  METALS. 

IRON. 
Iron — Cobalt, 

LITERATURE. 

Classen,  Ber.  deutsch.  chem.  Ges.,  27,  2060. 

The  two  metals  may  be  determined  by  electrolyzing  the 
solution  of  the  double  oxalates,  as  directed  under  Iron  (p. 
138),  weighing  the  iron  and  cobalt  together,  and  determin- 
ing the  former  volumetrically. 

After  weighing  the  iron  and  cobalt,  the  deposit  is  dis- 
solved in  dilute  sulphuric  acid  (dilute  sulphuric  acid  is  poured 
over  the  metals,  and  concentrated  acid  gradually  added,  so 
that  the  solution  becomes  heated),  and  the  iron  is  titrated  in 
the  platinum  dish  with  potassium  permanganate.  To  over- 
come the  red  color  of  cobalt  sulphate,  a  sufficient  amount  of 
nickel  sulphate  is  added  before  the  titration.  The  end  of 
the  reaction  is  easily  recognized. 

The  residue  of  cobalt  and  iron  may  also  be  dissolved  in 
hydrochloric  acid,  the  iron  oxidized  with  hydrogen  peroxide, 
and  titrated  with  stannous  chloride,  after  removing  the  excess 
of  hydrogen  peroxide  by  boiling. 

EXPERIMENT. 

Used  1  g  each  of  CoSO4.K,SO4.6H9O  and  Fe3(CaO4),. 
3K2C,O4.6H2O,  and  8  g  ammonium  oxalate.  Yolume  of 
liquid,  120  cc. 


192         QUANTITATIVE   ANALYSIS   BY    ELECTROLYSIS. 

Current    Electrode  Found, 

Density,     Tension,    Temp.         Time.  •&„   ,  c,n          Calculated.  Titrated. 

Amperes.     Volts. 

2.0-1.6    3.0-3.6    65-70°  1  hr.  40  m.     0.2658  g    0.1141  g  Fe*     0.1140  g  Fe 

0.1517  "Co 
0.2658  g 

1.55-1.43.2-3.6    62-65°   1  "    20"      0.2650"    0.1138  g  Fe      0.1140"" 

0.1517  "  Co 
0.2655  g 

1.0-0.85  2.85-3.1  60-65°   2  "    30"     0.2585"    0.1137  g  Fe       0.1140"" 

0.1451  "  Co 
0.2586  g 

0.5-0.4    2.0-2.7    60-67°  4  "    —         0.2593"     0.1136  g  Fe      0.1133"" 

0.1452  "  Co 
0.2588  g 

0.5-0.45  2.35-2.7  58-62°  4  "    —         0.2617"    0.1189  g  Fe      0.1141"" 

0.1477  "Co 
0.2616  g 


Iron— Nickel. 

LITERATURE  : 

Vortmann,  Monatshefte  f.  Chem.,  14,  536. 
Classen,  Ber.  deutsch.  chem.  Ges.,  27,  2060. 

The  method  of  determination  is  exactly  like  the  preceding. 
Iron  and  nickel  separate  in  the  form  of  a  beautiful  white  alloy 
scarcely  distinguishable  from  the  platinum.  This  alloy  resists 
strongly  the  action  of  acids,  and  is  only  very  slowly  attacked 
by  dilute  sulphuric  or  hydrochloric  acid. 

Since  the  precipitation  of  the  last  trace  of  nickel  takes 
place  very  slowly,  the  use  of  a  current  of  at  least  ND100  =  1 
ampere  is  to  be  recommended.  Toward  the  end  of  the  opera- 
tion the  current  strength  should  be  increased  to  1  ampere. 

To  determine  the  iron,  the  precipitate  in  the  dish  must  be 
heated  with  concentrated  hydrochloric  acid ;  and  if  the  iron 
is  to  be  titrated  with  permanganate,  the  solution  must  be 

*  The  numbers  placed  under  the  heading  "  Calculated  "  are  the  quanti- 
ties of  iron  and  cobalt  in  the  two  salts  taken,  which  were  separately  deter- 
mined by  electrolysis. 


IKON. 


193 


reduced  by  nascent  hydrogen.  It  is  more  simple  to  oxidize 
with  hydrogen  peroxide,  and,  after  removing  the  excess, 
titrate  the  ferric  chloride  with  stannous  chloride. 


EXPERIMENT. 


Used  1  g  each  of  NiSO4.(NH4)tSO4.6EtO  and  Fe,(C2O4)3. 
3KaC2O4.6H,O,  and  8  g  ammonium  oxalate.  Volume  of 
liquid,  120  cc. 


Current  Dens.,    Electrode 


Amperes. 
2.2-1.75 


Tens.,  Volts. 
3.45-4.0 


Temp. 

70-65° 


Time, 
hr.  m. 


Found. 
Fe4-Ni. 

0.2760  g 


2.0-1.75        335-3.9        69-67°      2—      0.2654 


1.1-0.7          2.6-3.1          65-71°      430      0.2675' 


0.5-0.4          2.6-3.0          68-71°      5—      0.2664 


Calculated. 

0.1135  gFe* 
0.1622"  Ni 
0.2757g 
0.1135  gFe 
0.1527"Ni 
0.2662  g 
0.1135  g  Fe 
0.1550"Ni 
0.2683  g 
0.2664  g 


Yortmann  adds  4-6  g  sodium  potassium  tartrate  and  an 
excess  of  sodium  hydroxide  to  the  solution,  and  precipitates 
the  iron  with  a  current  of  OT)100  —  0.3-0.5  ampere  in  three 
to  four  hours,  the  nickel  remaining  in  solution. 

Iron — Zinc. 
LITERATURE  : 

Vortmann,  Monatshefte  f.  Chem.,  14,  536. 

If  the  double  oxalates  of  iron  and  zinc  are  submitted  to 
electrolysis,  an  alloy  of  the  two  does  not  separate,  but  zinc, 
with  a  little  iron,  is  first  precipitated  on  the  negative  elec- 
trode. The  electrolysis  proceeds  very  satisfactorily,  and  the 


*The  numbers  placed  under  the  heading  "  Calculated"  are  the  quanti- 
ties of  iron  and  cobalt  in  the  two  salts  taken,  which  were  separately  deter- 
mined by  electrolysis. 


194         QUANTITATIVE   ANALYSIS   BY   ELECTROLYSIS, 

united  weight  of  the  two  metals  may  readily  be  determined 
if  there  is  less  than  one-third  as  much  zinc  as  iron  in  the  solu- 
tion. If  the  proportion  of  zinc  is  greater,  the  zinc  dissolves 
with  the  evolution  of  gas  as  the  action  proceeds,  and  a  pre- 
cipitate of  iron  oxide  is  formed. 

Vortmann  proposes  the  following  method  :  Several  grams 
of  potassium  sodium  tartrate  and  an  excess  of  a  10-20$  solu- 
tion of  sodium  hydroxide  are  added  to  the  solution  of  the 
metals,  and  the  liquid  is  electrolyzed  at  an  electrode  tension  of 
2  volts,  with  a  current  strength  of  KD100  =  0.07-0.1  ampere. 
It  is  best  to  raise  the  temperature  at  the  close  of  the  operation 
to  50-60°.  After  several  hours  the  iron  will  be  precipitated, 
the  zinc  remaining  in  solution. 

Iron — Manganese. 
LITERATURE  : 

Classen,  Ber.  deutsch.  chem.  Ges.,  18,  1787. 

A  solution  of  ammonium  oxalate  is  decomposed  by  elec- 
trolysis, as  stated  in  the  introduction,  mainly  into  hydrogen 
and  hydrogen  ammonium  carbonate.  The  latter  is  partly 
decomposed  into  ammonia,  most  of  which  remains  in  solution, 
and  carbon  dioxide.  In  the  electrolysis  of  a  hot  solution  of 
ammonium  oxalate,  the  ammonium  carbonate  produced  by  the 
current  is  partly  neutralized  as  a  result  of  dissociation  of 
ammonium  oxalate;  carbon  dioxide  is  rapidly  liberated  at  the 
positive  electrode. 

If  a  solution  of  the  double  oxalates  of  iron  and  manganese 
is  subjected  to  electrolysis  without  the  previous  addition  of  a 
great  excess  of  ammonium  oxalate,  the  characteristic  color  of 
permanganic  acid  appears  immediately  at  the  positive  elec 
trode,  manganese  dioxide  gradually  separates  at  the  posi- 
tive electrode,  and  iron  at  the  negative.  If  the  electrolysis 
is  conducted  under  these  conditions,  it  is  impossible  to  obtain 


IRON.  195 

a  quantitative  separation  of  the  two  metals,  since  the  manga- 
nese dioxide  carries  down  witli  it  considerable  quantities  of 
ferric  hydroxide.  The  complete  separation  of  the  metals  is 
possible  only  when  the  separation  of  the  manganese  dioxide  is 
delayed  till  most  of  the  iron  is  precipitated.  If  a  solution  of 
the  double  oxalates  of  iron  and  manganese,  which  contains  a 
great  excess  of  ammonium  oxalate,  is  electrolyzed  in  the  cold, 
the  greater  part  of  the  manganese  dioxide  is  precipitated  only 
after  most  of  the  ammonium  oxalate  is  decomposed.  In  this 
case,  however,  the  separation  of  the  manganese  dioxide  is  in- 
complete, because  by  the  action  of  the  current  a  considerable 
quantity  of  ammonium  carbonate  or  ammonia  is  produced 
which  acts  on  the  manganese  double  salt,  causing  a  portion  of 
the  precipitate  (a  mixture  of  dioxide  and  a  lower  oxide)  to  pass 
into  solution. 

The  rapid  dissociation  of  ammonium  oxalate  when  heated 
gives  a  simple  means  of  delaying,  or  entirely  preventing,  the 
formation  of  a  manganese  precipitate  during  electrolysis. 

The  double  oxalate  is  prepared  by  the  method  given  under 
iron,  with  the  difference  only  that  8  to  1 0  g  ammonium  oxa- 
late are  dissolved  in  the  liquid,  which  is  warmed  to  70°,  and 
electrolyzed  with  a  current  of  NT)100=  0.5  amp. 

When  the  reduction  is  complete,  the  solution  is  poured 
off,  the  dish  washed  repeatedly  with  water,  and  this,  together 
with  traces  of  the  dioxide  precipitate,  removed  by  alcohol ;  it 
is  sometimes  necessary  to  rub  the  dish  gently  with  the  finger. 

The  preceding  method  gives  satisfactory  results  when  the 
percentage  of  manganese  is  not  too  high.  For  the  analysis  of 
manganiferous  iron  (f erro-manganese,  for  example)  this  method 
has  no  practical  value,  since  the  per  cent  of  manganese  is  here 
required,  while  by  this  method  the  iron  is  determined  directly 
and  the  manganese  must  be  determined  in  the  liquid  from 
which  the  iron  has  been  separated. 


196         QUANTITATIVE  ANALYSIS   BY   ELECTROLYSIS. 

To  obtain  a  complete  separation,  the  solution,  containing 
suspended  manganese  dioxide,  is  heated  with  a  solution  of 
pure  potassium  or  sodium  hydroxide  in  a  porcelain  dish, 
till  the  ammonium  carbonate  produced  by  electrolysis  is  de- 
composed and  the  solution  no  longer  has  the  odor  of  ammonia ; 
and  then  sodium  carbonate  and  a  small  quantity  of  sodium 
hypochlorite,  or,  better,  hydrogen  peroxide,  are  added.  The 
manganese  dioxide  quickly  falls  to  the  bottom,  and  can  be 
filtered  off.  The  precipitate  is  best  washed  with  hot  water  to 
which  a  little  ammonium  nitrate  has  been  added,  and  is  either 
converted  into  mangano-manganic  oxide  (Mn3O4)  by  ignition, 
or,  better,  into  manganese  sulphate  (MnSO4). 

The  conversion  into  manganese  sulphate  is  accomplished 
by  moistening  the  precipitate  in  the  crucible  with  a  little  pure 
concentrated  sulphuric  acid,  arid  igniting  very  gently,  so  that 
the  bottom  of  the  crucible  is  barely  red. 

If  it  is  desired  to  determine  the  manganese  as  manganese 
sulphide,  the  solution  is  boiled  till  the  ammonium  carbonate 
is  decomposed,  the  remaining  ammonia  is  neutralized  with 
nitric  acid,  and  ammonium  sulphide  added  till  the  precip- 
itation is  complete.  The  manganese  sulphide  is  either  deter- 
mined as  such,  by  ignition  in  a  stream  of  hydrogen,  or,  more 
simply,  converted  into  manganese  sulphate  by  heating  with  a 
few  drops  of  sulphuric  acid. 

Iron — Aluminium. 
LITERATURE: 

Classen,  Ber.  deutsch.  chem.  Ges.,  18,  1795  ;  27,  2060. 
When  a  solution  containing  the  above-named  metals  and 
a  great  excess  of  ammonium  oxalate  is  electrolyzed  in  the 
cold,  iron  is  deposited  on  the  negative  electrode,  while  the 
aluminium  remains  in  solution  as  long  as  ammonium  oxalate  is 
present  in  the  solution  in  greater  proportion  than  the  ammo- 


IRON.  197 

mum  carbonate  formed  from  it.  If  a  precipitate  of  aluminium 
hydroxide  finally  appears,  it  is  only  when  the  solution  is  al- 
most free  from  iron.  A  small  portion  withdrawn  by  a  capil- 
lary tube  is  tested,  from  time  to  time,  with  ammonium  sul- 
phide or  another  reagent  already  mentioned,  and  the  current 
is  stopped  as  soon  as  no  reaction  is  obtained. 

The  process  is  as  follows:  The  aqueous  or  weakly  acid 
solution  (in  the  latter  case  neutralized  with  ammonia)  of  the 
sulphates  (the  chlorides  are  not  as  well  adapted  to  the  process) 
is  treated  with  ammonium  oxalate  in  excess,  and  enough  solid 
ammonium  oxalate  added  (with  gentle  warming  if  necessary) 
to  give  the  proportion  of  2— 3  g  ammonium  oxalate  to  0.1  g 
of  the  metals.  The  entire  volume  of  the  solution  should  be 
150-175  cc.  If  the  temperature  of  the  solution  is  not  over 
4:0°,  it  may  be  submitted  to  electrolysis  at  once,  since  it  grad- 
ually cools  under  the  action  of  a  current  of  the  given  strength. 

It  is  not  best  to  continue  the  action  of  the  current  longer 
than ,  is  necessary  to  reduce  the  iron ;  for,  otherwise,  a  large 
part  of  the  aluminium  is  precipitated  as  hydroxide,  and 
clings  so  closely  to  the  negative  electrode  that  it  cannot  be 
removed. 

In  such  a  case  it  is  necessary  to  bring  the  aluminium  hydrox- 
ide into  solution  by  acidifying  with  oxalic  acid,  and,  in  case 
too  much  acid  has  been  added,  to  pass  the  current  till  the 
last  traces  of  the  redissolved  iron  have  been  again  precipitated. 

The  oxalic  acid  is  poured  gradually  down  the  glass  which 
covers  the  platinum  dish,  without  interrupting  the  current, 
till  there  is  no  more  ebullition,  and  the  aluminium  precipitate 
is  dissolved. 

If  the  quantity  of  the  aluminium  is  not  greater  than  that  of 
the  iron,  the  method  gives  good  results  without  further  treat- 
ment. In  other  cases,  the  precipitate  of  aluminium  hydrox- 
ide is  dissolved,  without  interrupting  the  current,  by  careful 


198         QUANTITATIVE  ANALYSIS   BY    ELECTROLYSIS. 

addition  of  oxalic  acid,  and  the  electrolysis  repeated  until 
the  iron  is  completely  precipitated.  To  determine  the  alu- 
minium in  the  solution  poured  off  from  the  iron,  it  is  heated 
in  a  porcelain  dish  till  the  ammonia  is  driven  off,  filtered, 
and  the  aluminium  hydroxide  converted,  by  ignition,  into 

A1.0.- 

EXPERIMENT. 

Usedl  g  each  of  Fe2(C2O4)3.3K2C2O4.6H2O  and  A12(SO4)3. 
K2SO4.24rH2O,  and  8  g  ammonium  oxalate.  Yolume  of 
liquid,  120  cc. 


Current 
Density, 
Amperes. 

Electrode 
Tension, 
Volts. 

Temp. 

Time 
hr.  m. 

Found, 

g- 

Taken, 
£• 

1.95-1.6 

4  3  -4.4 

31-42° 

2  35 

0.1143  Fe 

0.1135  Fe 

1.65-1.35 

3.8  -4.1 

30-48° 

3  — 

0.1159  " 

0.1150   " 

1.00-0.84 

3.55-3.8 

31-36° 

4  30 

0.1138  " 

0.1135   " 

0.50-0.42 

2.75-3.1 

30-32° 

5  40 

0.1139  " 

0.1135   " 

In  order  to  avoid  the  separation  of  aluminium  hydroxide 
(small  quantities  of  which  often  adhere  to  the  iron)  strong 
currents,  which  raise  the  temperature  of  the  solution,  should 
not  be  used. 

The  effect  of  strong  currents  and  high  temperatures  is 
illustrated  in  the  above  experiment. 

Iron — Uranium. 

The  separation  of  iron  from  uranium  depends  upon  the 
same  principle  as  the  separation  from  aluminium.  It  is  nec- 
essary to  have  a  great  excess  (8  g)  of  ammonium  oxalate  present 
in  the  solution,  in  order  to  retain  the  uranium  in  the  form 
of  the  double  salt  until  all  of  the  other  metals  are  reduced. 
The  process  is  conducted  in  the  same  manner  as  in  the  separa- 
tion of  aluminium  from  iron.  When  a  strong  current  is 
employed,  especially  when  there  is  an  insufficient  quantity  of 
ammonium  oxalate  present,  it  may  happen  that,  as  a  result  of 


IRON.  199 

the  decomposition  of  the  hydrogen  ammonium  carbonate  by 
the  heat  produced,  the  uranium  is  precipitated  as  hydroxide. 

The  uranium  solution,  after  the  other  metals  have  been 
separated,  is  freed  from  oxalic  acid  by  further  electrolysis,  and 
finally  the  ammonium  carbonate  is  decomposed  by  heating. 

To  bring  the  finely  divided  precipitate  of  uranium  hydrox- 
ide into  suitable  condition  for  filtration,  nitric  acid  is  added,  the 
solution  is  heated  till  the  precipitate  is  wholly  dissolved,  and 
ammonia  is  added  to  reprecipitate  the  hydroxide.  The  pre- 
cipitate is  converted  into  uranium  oxide  by  ignition  in  a 
stream  of  hydrogen. 

Iron — Chromium. 
LITERATURE  : 

Classen,  Ber.  deutsch.  chem.  Ges.,  27,  2060. 

If  a  solution  which  contains  an  excess  of  ammonium  oxa- 
late,  and  chromium  as  sesquioxide,  that  is,  as  chromium 
ammonium  oxalate,  be  submitted  to  electrolysis,  all  of  the 
chromium  is  converted  into  a  chromate.  If  iron  is  also  pres- 
ent, it  is  precipitated  in  the  metallic  state  on  the  negative 
electrode ;  the  metal  has  a  peculiarly  characteristic  lustre. 

When  the  precipitation  is  complete,  the  liquid  is  poured 
off  from  the  precipitated  metal  and  is  boiled  to  decompose 
ammonium  carbonate,  and  the  chromic  acid  reduced  by  boiling 
with  hydrochloric  acid  and  alcohol.  The  chromium  is  then 
precipitated  as  hydroxide  with  ammonia. 

The  hydroxide  is  converted  into  Cr3O,  in  the  usual  mariner, 
and  weighed. 

EXPERIMENT. 

A.  Used  1  g  each  of  Fe,(C2O4)s.3K,CaO4.6HaO  and 
3K,C,O4.Cr,(C,O4)3.6HaO,  and  8  g  ammonium  oxalate.  So- 
lution diluted  to  120  cc. 


200         QUANTITATIVE   ANALYSIS    BY    ELECTROLYSIS. 


Amperes. 

Electrode 
Tension, 
Volts. 

Temp, 

Time, 
hr.  m. 

Found 
Fe. 

Taken 
Fe. 

2.00-1.60 

3.4-3.6 

62-68° 

^  

0.1123  g 

0.1120  g 

1.60-0.95 

3.2-3.8 

66-68° 

5  - 

0.1135  " 

0.1135  " 

1.95-1.50 

3.3-3.7 

62-65° 

3  — 

0.1130  " 

0.1135  " 

B.  Used  2  g  chrome  alum,  1.5890  g  ferrous  ammonium 
sulphate,  and  8  g  ammonium  oxalate. 

1.5  3  65°  4  14.19$  Fe        14.28$  Fe 

C.  Used  2  g  chrome  alum,  1  g  Fe2(C,O4)3.3K2CaO4.6H2O, 
and  8  g  ammonium  oxalate. 

1.50-1.60        3.0-3.2  65°         4  11.35*  Fe        11.40$  Fe 

Iron  —  Aluminium  —  Chromium. 
LITERATURE  : 

Classen,  Ber.  deutsch.  chem.  (res.,  14,  2771. 

The  separation  is  performed  as  above.  To  separate  the 
aluminium  from  chromium,  the  solution  poured  off  from  the 
precipitated  metals  is  boiled  till  it  has  only  a  weak  odor  of  am- 
monia, the  aluminium  hydroxide  filtered  off,  and  the  chro- 
mium precipitated  as  above. 

Iron—  Chromium—  Uranium. 

LITERATURE  : 

Classen,  Ber.  deutsch.  chem.  Ges,  14,  2771;  17,  2483. 

The  separation  is  accomplished  by  the  precipitation  of 
iron  as  metal,  from  the  double  oxalate  solution,  and  the  oxi- 
dation of  chromium  to  chromic  acid  by  the  current.  Uranium 
is  separated  as  hydroxide,  while  chromium  remains  in  solution 
as  ammonium  chromate.  To  accomplish  the  quantitative  sep- 
aration of  chromium  from  uranium,  the  electrolysis  must  be 
continued  till  the  oxalic  acid  is  completely  oxidized. 


IRON.  201 

The  solution  is  boiled  to  decompose  the  resulting  ammo- 
nium carbonate,  and  allowed  to  stand  six  hours.  The  chromi- 
um is  determined,  as  above,  in  the  filtrate  from  the  uranium. 

Iron — Beryllium, 

LITERATURE : 

Classen,  Ber.  deutsch.  chem.  Ges.,  14,  2771, 

The  separation  of  these  two  metals  offers  no  difficulties 
whatever  if  the  soluble  double  salts  with  ammonium  oxalate 
are  prepared,  and  if  care  is  taken  to  have  an  excess  of  ammo- 
nium oxalate  present.  The  iron  is  precipitated  according  to 
the  directions  given  under  the  separation  of  aluminium  from 
iron. 

Strong  currents  are  not  advisable  lest  the  solution  become 
heated,  and  thus  the  ammonium  carbonate,  which  holds  the 
beryllium  in  solution,  be  decomposed.  The  beryllium  hy- 
droxide may,  in  any  case,  begin  to  precipitate  before  the  iron 
is  fully  deposited.  The  determination  of  beryllium  in  the 
solution  poured  off  from  the  iron  is  very  simple;  the  solution 
is  boiled  to  decompose  the  hydrogen  ammonium  carbonate, 
and  the  heating  continued  till  the  solution  has  only  a  weak 
odor  of  ammonia.  The  beryllium  hydroxide  is  filtered,  washed 
with  hot  water,  and  converted  into  BeO  by  ignition  in  a 
platinum  crucible. 

Iron — Beryllium — Aluminium, 

LITERATURE  : 

Classen,  Ber.  deutsch.  chem.  Ges.,  14,  2771. 

The  process  is  precisely  like  the  foregoing.  When  the 
iron  is  reduced,  the  solution  is  poured  into  a  second  platinum 
dish,  and  the  action  of  the  current  is  continued  till  all  the 


202        QUANTITATIVE  ANALYSIS   BY   ELECTROLYSIS. 

oxalic  acid  is  decomposed,  and  the  aluminium  is  precipitated 
as  hydroxide.  The  beryllium  is  precipitated  from  the  filtrate 
as  hydroxide  by  boiling. 

It  is  advisable  to  redissolve  the  aluminium  hydroxide, 
to  convert  it  again  into  the  double  oxalate,  and  to  repeat  the 
electrolysis. 

Iron — Copper. 
LITERATURE  : 

Vortmann,  Monatshefte  f.  Chem.,  14,  536. 
Classen,  Ber.  deutsch.  chem  Ges.,  27,  2060. 

The  separation  may  be  accomplished  according  to  the 
method  given  by  Luckow  (p.  156),  if  the  operation  is  conducted 
at  ordinary  temperatures.  *  To  determine  the  iron  in  the  solu- 
tion from  which  the  copper  has  been  removed,  it  is  evaporated 
to  dryness  with  the  addition  of  sufficient  sulphuric  acid  to 
convert  the  iron  into  sulphate,  and  the  double  oxalate  is  pre- 
pared by  the  method  given  on  page  138. 

EXPERIMENT. 

Used  about  1  g  each  of  copper  sulphate  and  ferrous  am- 
monium sulphate  and  5  cc  nitric  acid  (sp.  g.  1.35).  Volume 
of  liquid,  120  cc. 

Current  Dens.,  Electrode         Tomn          Time,         Found  Taken 

Amperes.    Tens.,  Volts.         ^mp'          hr.  m.  Cu.  Cu. 

1.0-0.9        3.0-3.3        19-32°        4  —       0.2518  g        0.2528  g 
1.1-1.0        2.6-3.2        18-32°        3  30       0.2430 "        0.2450 " 

The  free  sulphuric  acid  in  the  decanted  liquid  was  neu- 
tralized with  ammonium  hydroxide,  and  8  g  ammonium  oxalate 
were  added. 

Current  Dens.,   Electrode  T  Time,  Found              Taken 

Amperes.      Tens.,  Volts.  hr.  m.  Fe.                    Fe. 

1.30-0.8        2.7-4.5  31-42°  3—  0.1416  g  0.1406  g 

145-11        3.0-3.5  60°  330  0.1438"  0.1435" 


IRON.  203 

A  similar  separation  may  also  be  carried  out  in  the  pres- 
ence of  sulphuric  acid  instead  of  nitric  acid.  Three  cubic 
centimeters  of  the  concentrated  acid  are  used,  the  other  con- 
ditions being  the  same. 

Current  Dens.,     Electrode          T«m^  Time,          Found  Taken 

Amperes.       Tens ,  Volts.  hr.m.  Cu.  Cu. 

1.05-1.20        3.0-2.85        22-30°        210       0.2534  g        0.2539  g 
1.00-0.95        2.5-2.45        56-59°        2—       0.2504"        0.2510" 

The  determination  of  the  iron  was  conducted  as  before. 

Fe.  Fe. 

1.55-1.32        3.4-3.8          33-40°        4—        0.1419  g        0.1421  g 
1.60-1.40        3.0-3.5          61-64°        3—        0.1625"        0.1675" 

The  separation  of  iron  and  copper  may  be  performed  if 
the  copper  is  precipitated  from  a  hot  solution  of  the  double 
oxalate  containing  free  oxalic,  tartaric,  or  acetic  acid.  A 
saturated  solution  of  oxalic  acid  is  used,  and  one  of  tartaric 
acid  which  contains  6  g  acid  in  every  100  cc. 


EXPERIMENT. 

Used  about  1  g  each  of  copper  sulphate  and  ferric  salt, 
6  g  ammonium  oxalate.  The  copper  must  be  washed  without 
interrupting  the  current. 


Amperes.  Volts.  Temp.          Time. 

1.1-1.0        2.95-3.5         51-62°        3  hr.        0.2525  g        0.2528  g 
0.7-0.7        3.20-285       62°  3"          0.2532"        0.2530" 

The  iron  was  determined  in  the  solution  which  was  poured 
off  from  the  copper,  the  free  acid  being  first  neutralized  with 
ammonium  hydroxide. 

Fe.  Fe. 

1.4-1.3        3.0-3.2        68-70°        2}  hr.        0.1431  g        0.1435  g 
1.0-0.9        3.1-3.3        30-40°        3     "          0.1425"        0.1429" 


204         QUANTITATIVE   ANALYSIS   BY   ELECTEOLYSIS. 

Yortmann  dissolves  the  oxides  of  both  metals  in  an  am- 
moniacal  solution,  to- which  are  added  several  grams  of  ammo- 
nium sulphate,  and  electrolyzes  with  a  current  of  OTD100  = 
0.1-0.6  ampere.  Only  copper  is  precipitated,  the  ferric 
hydroxide  remaining  unaltered  in  solution. 

Iron — Lead. 

The  separation  is  based  on  the  separation  of  lead  as  per- 
oxide in  the  presence  of  nitric  acid  (p.  168).  The  iron  is 
.determined  as  above. 

COBALT. 

Cobalt — Zinc. 
LITERATURE  I 

Vortmann,  Elektrochem.  Zeit.,  1,  6. 

Smith  and  Wallace,  Journ.  of  anal.  Chem.,  1893,  p.  183. 

According  to  Yortmann,  an  excess  of  a  10-20$  solution 
of  sodium  hydroxide  is  added  to  the  solution  containing  the 
metals.  Several  grams  of  sodium  potassium  tartrate  are  then 
added  and  the  electrolysis  is  conducted  with  a  current  of 
NI)100  =  0.07-0.1  ampere  and  an  electrode  tension  of  2 
volts.  The  cobalt  is  precipitated,  but  the  addition  of  potas- 
sium iodide  is  necessaiy  in  order  to  prevent  the  separation  of 
cobaltic  oxide  at  the  anode. 

Cobalt — Aluminium. 

The  method  is  carried  out  similarly  to  that  of  iron  from 
aluminium. 

Cobalt— Uranium  ;  Cobalt— Chromium  ;  Cobalt— Uranium — Chromium 

The  methods  employed  are  similar  to  those  of  the  corre- 
sponding separations  from  iron  (p.  200). 


COBALT.  205 


Cobalt—  Copper. 


LITERATURE  : 


Classen,  Ber.  deutsch.  chem.  Ges.,  27.  2060. 
Rudorff,  Zeit.  f.  angew.  Chem.,  1894,  p.  388. 
Warwick,  Zeit.  f.  anorg.  Chem.,  1,  299. 

The  separation  of  these  two  metals  can  only  be  satisfac- 
torily carried  out  by  the  electrolysis  of  solutions  containing 
oxalic,  tartaric,  or  dilute  acetic  acid,  at  a  temperature  of  50- 
60°,  and  at  an  electrode  tension  of  not  less  than  1.1  or  more 
than  1.3  volts.  In  order  to  have  the  tension  constant  and  to 
be  able  to  regulate  it  conveniently,  it  is  best  to  insert  the  wire- 
gauze  resistance  described  on  page  113  in  the  main  circuit, 

EXPERIMENT. 

Used  1  g  copper  sulphate,  1  g  cobalt  ammonium  sulphate,, 
and  6  g  ammonium  oxalate. 


trorle  Tension, 
Volts. 

Temp. 

Time, 
hr.        m. 

Found* 
g  Cu.              %  Cu. 

1.24-1.30 

50-60° 

3 

50 

0.2602 

25.36 

1.20-1.35 

50-60° 

3 

30 

0.2531 

25.29 

1.20-1.29 

50-60° 

4 

— 

0.2522 

25.28 

Cobalt—  Bismuth. 
LITERATURE  : 

Smith  and  Wallace,  Journ.  of  Anal.  Chem.,  1893,  p.  183. 
Smith  and  Moyer,  Zeit.  f.  anorg.  Chem.,  4,  268. 

According  to  Smith  and  Wallace,  and  also  Smith,  and 
Moyer,  a  separation  of  these  metals  may  be  satisfactorily  con- 
ducted in  a  solution  containing  nitric  acid.  Since,  however, 
the  required  conditions  of  experiment  are  not  given  in  the 
respective  publications,  the  methods  will  be  here  omitted. 


*  [Theory  25  33g  Cu.  ] 


206         QUANTITATIVE   ANALYSIS   BY    ELECTROLYSIS. 


Cobalt— Lead. 

The  solution,  to  which  nitric  acid  has  been  added,  is 
electrolyzed  (see  Lead). 

Cobalt — Mercury. 
Similar  to  the  above. 

NICKEL. 
Nickel — Manganese. 

What  has  been  said  with  reference  to  the  separation  of 
iron  from  manganese  applies  also  to  the  separation  of  nickel 
from  manganese. 

Nickel — Aluminium. 

Similar  to  the  separation  of  iron  from  aluminium. 

Nickel — Uranium  ;  Nickel — Chromium. 
See  Iron  (pp.  198-199). 

Nickel— Copper. 
LITERATURE  I 

Classen,  Ber.  deutsch.  chem.  G-es.,  27,  2060. 

The  separation  takes  place  under  the  same  conditions  as 
the  separation  of  cobalt  from  copper. 

If  1  g  each  of  copper  sulphate  and  nickel  sulphate  are 
taken,  6  g  ammonium  oxalate  are  required.  Larger  quantities 
of  metal  require  correspondingly  greater  quantities  of  the 
ammonium  oxalate. 


NICKEL.  207 


EXPERIMENT. 

Elec.Tens.,       Time,  Found* 

Volts.  hr.  in.          g  Cu.  %  Cu.  Remark. 

1.11-1.3        3  50        0.2552        25.40 

1.20-1.3         3—         0.2559         25.37         Acidified  with  oxalic  acid. 

1.20-1.3         3  30         0.2591          25.38          Acidified  with  tartaric  acid. 

J  Acidified  with  acetic  acid.    The 
1     copper  contained  nickel. 


1  Q4_1  A^       Q   ^O         n  9^7Q 


1.20-1.6         350         0.2595         25.33         The  copper  contained  nickel. 
Nickel— Lead. 

The  separation  corresponds  to  the  method  given  under 
Cobalt. 

Nickel — Mercury. 
LITERATURE  I 

Kudorff,  Zeit.  f.  angew.  Chem.,  1894,  p.  388. 
Smith,  Am.  Chem.  Jouru.,  12,  104. 
Heidenreich,  Ber.  deutsch.  chein.  Ges.,  28,  1585. 

The  method  for  the  separation  of  these  two  metals  is 
similar  to  that  of  cobalt  from  mercury.  According  to  the 
statements  of  Smith,  the  separation  may  be  carried  out  from 
a  solution  of  the  double  cyanides.  Heidenreich,  who  de- 
termined in  the  Aachen  laboratory  the  proper  conditions 
of  experiment,  found  that  only  the  mercury  is  precipitated 
when  the  tension  at  the  electrodes  is  1.2-1.6  volts. 


EXPERIMENT. 

Used  about  1  g  nickel  ammonium  sulphate  and  3  g  potas- 
sium cyanide. 


Taken 
f  HgCI2. 

Current  Density, 
Amperes. 

El.  Tension, 
Volts. 

Time. 

Found  t 
per  cent  Hg. 

03687 

0  08-0.03 

1.2-1.6 

5£  hr. 

73.65 

0.3702 

0.05-0.93 

1.4-1.5 

overnight 

73.62 

0.3000 

0.05-0.03 

1.4-1.5 

" 

73.66 

•  [Theory  25.33^  Cu.]  f  [Theory  73.80£  Hg.] 


208         QUANTITATIVE   ANALYSIS   BY    ELECTROLYSIS. 

ZINC. 
Zinc — Manganese. 

This  separation,  similar  to  that  of  copper  from  cobalt, 
takes  place  from  hot  solutions  containing  free  oxalic  acid, 
which  prevents  the  separation  of  manganese  peroxide. 

Zinc — Aluminium. 

Conditions  similar  to  the  above. 

Zinc — Copper. 
LITERATURE  I 

Riidorff,  Zeit.  f.  angew.  Chem.,  1893,  p.  452. 

Smith  and  Wallace,  Journ.  of  Anal.  Chem.,  1893,  p.  183. 

Heidenreich,  Ber.  deutsch.  chem.  Ges.,  28,  1585. 

For  this  separation,  Smith  and  Wallace  recommend  the 
precipitation  of  the  copper  from  a  solution  to  which  nitric 
acid  has  been  added.  Heidenreich,  who  determined  in  the 
Aachen  laboratory  the  proper  conditions  of  experiment,  found 
that  if  the  solution  contains  about  4  cc  nitric  acid  (sp.  g.  =  1 . 3) 
to  120  cc  of  liquid,  and  the  tension  of  1.4  volts  is  not 
exceeded,  the  copper  only  is  precipitated.  The  greater  part 
of  the  copper  separates  in  a  short  time,  but  the  precipitation 
of  the  last  trace  proceeds  very  slowly.  The  analysis  therefore 
requires  from  18  to  20  hours. 

EXPERIMENT. 

Used  copper  sulphate  (containing  25.29$  Cu)  to  which 
was  added  0. 8  g  zinc  ammonium  sulphate. 


ZINC. 


Taken 
CuS04.5H20 

g- 

Current 
,      Density, 
Amperes. 

Electrode 
Tension, 
Volts. 

Time, 
hr.     m. 

Found 
Cu, 

% 

0.4476 

0.2 

1.00-1.10 

6 

30 

24.31 

0.3857 

0.2-0.3 

1.00-1.20 

8 

— 

25.00 

0.4244 

0.2 

1.00-1.15 

15 

30 

25.19 

0.4689 

0.2 

1.00-1.15 

15 

30 

25.25 

0.4728 

0.2-0.15 

1.00-1.20 

18 

— 

25.25 

0.5049 

0.20-0.15 

1.13 

18 

— 

25.31 

0.4660 

0.5 

1.20 

2 

— 

25.22 

0.4775 

1.05-0.9 

1.50 

2 

— 

25.84 

)  contained 

0.4826 

1.00-0.8 

1.35-1.98 

18 

— 

25.80 

)      zinc 

0.4576 

0.50-0.4 

1.15-1.23 

6 

30 

25.19 

Zinc — Cadmium. 
LITERATURE. 

Smith,  Am.  Chem.  Journ.,  11,  352. 
Yver,  Bull.  Soc.  Chern.,  34,  18. 
Eliasberg,  Zeit.  f.  anal.  Chem.,  24,  550. 
Smith  and  Knerr,  Am.  Chem.  Journ.,  8,  210. 

A.  Yver  recommends  the  use  of  a  solution  of  the  ace- 
tates or  sulphates  treated  with  an  excess  of  sodium  acetate 
and  a  few  drops  of  acetic  acid ;  the  electrolysis  to  be  con- 
ducted hot,  using  two  Daniell  cells. 

In  the  laboratory  of  the  Technical  High  School  in  Munich 
the  following  directions  are  given  for  Tver's  method  :  To  the 
sulphuric  acid  solution  of  the  two  metals  add  sodium  hydrox- 
ide solution  until  a  permanent  precipitate  is  obtained,  dissolve 
the  precipitate  in  the  smallest  possible  quantity  of  dilute  sul- 
phuric acid,  dilute  the  solution  to  about  70  cc,  and  reduce  the 
cadmium  with  a  current  of  ND100  =  0.07  ampere.  When  the 
greater  part  of  the  metal  is  precipitated,  neutralize  the  free 
sulphuric  acid  with  sodium  hydroxide,  add  3  g  sodium  acetate, 
heat  to  about  45°,  and  subject  to  the  action  of  a  current  of 
ND)00  =0.3  ampere.  The  latter  direction  assumes  that  the 
electromotive  force  is  not  over  3.6  volts;  if  more,  it  is  to  be 
reduced  to  about  2.4  volts. 


210        QUANTITATIVE  ANALYSIS   BY   ELECTROLYSIS. 

Zinc— Lead. 

The  separation  is  conducted  from  a  nitric  acid  solution, 
the  lead  being  precipitated  as  peroxide  (see  Lead,  p.  168). 
To  determine  the  zinc,  it  is  converted  into  the  sulphate  in 
the  manner  described  under  the  separation  of  iron  from  copper 
on  page  202,  and  is  precipitated  by  the  method  given  on 
page  146. 

Zinc— Silver. 
LITERATURE : 

Smith  and  Wallace,  Zeit.  f.  Elektrochemie,  2,  312; 

Journ.  of  Anal.  Chem.,  1892,  p.  87. 
Heidenreich,  Ber.  deutsch.  chem.  Ges.,  28,  1585. 

The  separation,  according  to  Smith  and  Wallace,  is  con- 
ducted from  a  solution  of  the  double  cyanide.  The  proper 
experimental  conditions  were  acertained  by  Heidenreich  in  the 
Aachen  laboratory,  with  the  result  that  the  separation  was 
best  carried  out  at  a  temperature  of  60-70°,  and  with  a  ten- 
sion at  the  electrodes  of  1.9-2  volts. 

EXPERIMENT. 

Taken  Current  Density,      Electrode  Tens.,  rpPTnn  Timfk  Found* 

gAgN03.  Amperes.  Volts.  %  Ag. 

0.4046  0.05  1.9-2.03  60°  28  hr.  63.34 

0.4149  0.03  2.1-2.05  "  22   "  63.31 

0.3260  0.08  1.9  "•  16   "  63.23 

0.3739  0.08-0.05  3.0-2.15  "  15   "  63.31 

0.2949  0.05-0.02  1.8-2.05  "  6£   "  63.36 

Zinc— Mercury. 
LITERATURE  I 

Smith  and  Wallace,  Zeit.  f.  Elektrochemie,  2,  312. 
Heidenreich,  Ber.  deutsch.  chem.  Ges.,  28,  1585. 

Smith  and  Wallace  conduct  the  separation  from  a  solution 
of  the  double  cyanide.  According  to  the  experiments  carried 

*  [Theory  63.52$  Ag.J 


MANGANESE.  211 

out  in  the  Aachen  laboratory  by  Heidenreich,  the  mercury 
is  precipitated  free  from  zinc. 

In  performing  the  experiments,  Heidenreich  observed 
also  that  the  dishes  used  suffer  severely  from  the  combined 
action  of  the  mercury  and  potassium  cyanide  on  the  platinum. 

EXPERIMENT. 

Taken  Current  Density,      Elec.  Tension,  m-^  Found* 

gHgCl2.     gKCN.  Ampere.     '  Volts.  £Hg. 

0.2501        2-3  0.08-0.04  1.65-1.69  5  hr.  73.61 

0.2655        2-3  0.03  1.75  14  "  73.51 

MANGANESE. 
Manganese — Copper. 

The  separation  is  conducted  similarly  to  that  of  copper 
from  cobalt.  The  copper  is  precipitated  from  a  hot  solution 
containing  free  oxalic  acid  which  prevents  the  separation  of 
manganese  peroxide.  The  liquid  containing  the  manganese 
is  poured  off  from  the  copper.  Generally  this  is  not  suited 
for  direct  electrolytic  determination,  since  the  substances 
previously  added  interfere  with  the  precipitation  of  the 
manganese,  and  the  volume  of  the  liquid  has  become  too 
great  as  a  result  of  washing  the  copper  without  interrupt- 
ing the  current.  According  to  the  directions  of  Jannasch 
the  manganese  is  then  precipitated  with  ammonia  and  hydro- 
gen peroxide.  The  precipitate  is  allowed  to  settle  and  the 
solution  is  filtered.  The  precipitate  is  dissolved  in  a  mixture 
of  5  cc  of  acetic  acid,  5  cc  hydrogen  peroxide,  and  25  cc 
water,  and  this  solution  is  submitted  to  electrolysis  after  the 
excess  of  hydrogen  peroxide  has  been  removed  with  chromic 
oxide.  The  same  method  is  employed  when  the  solution 
contains  manganese  chloride,  since  the  presence  of  the 
chlorine  also  interferes  with  the  separation  of  the  peroxide. 

The  following  experiment  was  performed  as  above  directed 
by  Dr.  Oarl  Engels  in  the  Aachen  laboratory. 

*  [Theory  73.80#Hg.] 


212        QUANTITATIVE  ANALYSIS   BY   ELECTROLYSIS. 


EXPERIMENT. 

The  solution  contained  (NH4)2MnCl4.7H2O. 

(NH4)2MnCl4.7HaO     Current     Electrode  „,.  Found* 

taken,  Density,     Tension,       Temp.  »».     ®f  Mn3O4, 

g.  Amp.  Volts.  g.  % 

0.8619  0.63  2.8  80°  1    45  0.2153    24.98 

0.9550  0.62  2.8  82°  1    45  0.2385    24.98 

0.9562  0.70  2.9  83°  1    45  0.2394    25.03 

1.0131  0.72  3.1  80°  1     30  0.2536    25.03 

0.8580  0.80  3.1  80°  1     15  0.2151     25.01 

1.1383  0.78  3.1  85°  1     15  0.2848    25.02 


Manganese — Cadmium. 

This  is  conducted  similarly  to  the  separation  of  manganese 
and  copper,  and  the  manganese  is  determined  according  to  the 
directions  of  Engels,  which  are  given  above. 

COPPER. 

Copper — Cadmium. 
LITERATURE  I 

Freudenberg,  Zeit.  f.  phys.  Chem.,  12,  97. 
Heidenreich,  Ber.  deutsch.  chem.  Ges.,  28,  1585. 
Smith  and  Wallace,  Journ.  of  Anal.  Chem.,  1893,  p.  183. 
Smith  and  Moyer,  Zeit.  f.  anorg.  Chem.,  1,  299. 

According  to  the  statements  of  Freudenberg,  the  two 
metals  may  be  separately  precipitated  from  a  sulphuric  acid 
solution  (10—20  cc.  of  dilute  sulphuric  acid)  by  a  variation  of 
the  tension.  With  a  tension  of  2  volts  the  copper  is  precipi- 
tated, all  the  cadmium  remaining  in  solution. 

Heidenreich  tested  this  method  in  the  Aachen  laboratory, 
and  found  that  the  separation  is  best  conducted  with  a  tension 
not  exceeding  1.85  volts. 

*  [It  seems  probable  that  the  salt  taken  was  not  pure.     MnCl8.2NH4Cl. 
7HaO  contains  21.270  of  Mn3O4.— Trans.] 


COPPER.  213 


EXPERIMENT. 

The  volume  of  the  liquid  was  120  cc,  containing  15  cc 
dilute  sulphuric  acid  (sp.  gr.  =  1.09). 

Taken  Current  Density  Electrode          „,.  Found* 

CuS04.5H.jO,  CdS04.8H2O,  ND100,  Tension,  LF^'  Cu, 

g.  g.  Amperes.  Volts.  % 

0.7078  0.4  0.07-0.05  1.7-1.76  24  25.27 

Experiments  in  which  it  was  attempted  to  replace  the 
sulphuric  acid  by  nitric  acid  yielded  no  satisfactory  results. 

Copper — Lead. 
LITERATURE : 

Classen,  Ber.  deutsch.  chem.  Ges.,  27,  2060. 
Nissenson,  Zeit.  f.  augew.  Chem.,  1893,  p.  452. 

To  separate  copper  from  lead,  20  cc  of  nitric  acid  (sp.  g. 
1.35)  are  added  to  the  solution,  which  is  then  diluted  to 
75  cc,  warmed  and  electrolyzed  with  a  current  of  1.1-1.2 
amperes  (corresponding  to  ND100  =  1.5-1.7  amperes).  At  the 
end  of  one  hour  the  greater  part  of  the  lead  has  separated  as 
peroxide  (98-99$  when  not  more  than  0.5  g  is  present  in  the 
solution),  and  the  current  is  then  interrupted,  no  trace  of 
copper  as  yet  appearing  at  the  cathode.  The  liquid  is  then 
transferred  to  a  second  tared  dish,  the  lead  peroxide  is  washed 
with  water,  and  after  drying  is  weighed.  The  washings  from 
the  lead  peroxide  are  added  to  the  copper  solution,  which  is 
then  treated  with  ammonium  hydroxide  until  the  well-known 
deep  blue  color  appears,  and  about  5  cc  nitric  acid  are  added. 
The  platinum  dish  is  connected  with  the  negative  pole  of  the 
source  of  current,  and  one  of  the  perforated  platinum  bucket 
electrodes,  described  by  the  author,  is  employed  as  anode  to 

*  [Theory  25  33^  Cu.] 


214       >  QUANTITATIVE   ANALYSIS   BY   ELECTROLYSIS. 

take  up  the  remainder  of  the  lead  peroxide.  This  electrode 
should  have  a  roughened  surface.  It  is  weighed  before  the 
experiment.  After  the  solution  has  cooled,  it  is  diluted  to 
120-150  cc,  and  electrolyzed  with  a  current  of  OT3100  —  1.0— 
1.2  amp.  At  the  end  of  3  to  4  hours  the  copper  (if  about 
0.25  g  is  present),  together  with  the  rest  of  the  lead,  is  pre- 
cipitated. 

This' method,  which  is  of  great  value  in  technical  work,  is 
not  only  rapid  (4—5  hours  as  compared  to  14  hours  or  more), 
but  allows  of  the  complete  precipitation  of  both  metals, 
irrespective  of  the  relative  quantities  present. 

When  this  method  is  employed  for  the  analysis  of  sub- 
stances containing  sulphur,  the  lead  sulphate  resulting  from 
the  oxidation  is  troublesome.  The  operation  of  dissolving 
this  may  often  require  more  time  than  the  analysis  itself. 

Accordingly,  if  lead  sulphate  is  formed,  either  as  a  result 
of  the  oxidation  of  sulphur  or  of  double  decomposition  between 
lead  nitrate  and  copper  sulphate,  a  slight  excess  of  ammonia 
is  added  and  the  solution  is  warmed  for  several  minutes. 
The  dense  lead  sulphate  is  hereby  converted  into  porous  lead 
hydroxide,  The  liquid  is  poured  little  by  little  into  the 
platinum  dish,  which  contains  about  20  cc  of  warm  nitric 
acid,  and  constantly  stirred  with  the  electrode.  The  lead 
sulphate  which  reappears  either  dissolves  immediately,  or  if 
the  quantity  is  large  the  greater  part  of  it  goes  immediately 
into  solution,  and  the  remainder  disappears  on  warming  for  a 
short  time.  The  vessel  in  which  the  decomposition  of  the 
lead  sulphate  is  conducted  is  first  washed  with  a  little  nitric 
acid  and  then  with  water. 

EXPERIMENT. 

Usetf  about  1  g  each  of  lead  nitrate  and  copper  sulphate,, 
and  20  cc  nitric  acid. 


COPPER.  215 


Current 
Density, 
Amperes. 

Electrode  Tension, 
Volts. 
Beginning.         End. 

Temp. 

Time, 
hr. 

Found 
PbOa, 
g- 

Taken 
PbO,, 

g- 

1.1  -1.1 

1.4 

1.4 

60-63° 

1 

0.7266 

0.7260 

1.55-1.45 

1.4 

1.4 

66-72° 

1 

0.7310 

0.7303 

The  liquid  was  poured  off  from  the  lead  peroxide,  made 
alkaline  with  ammonia,  and  5  cc.  nitric  acid  were  added. 
The  copper  was  then  separated  by  electrolysis. 

Current  Electrode  Timp  Found  Taken 

Density,  Tension,  Temp.  ^r c'  Cu,  Cu, 

Amperes.  Volts.  g.  g. 

1.1-1.0  2.2  -2.5  25-30°  5  0.2490  0.2495 

1.0-0.95  2.25-2.3  30-32°  5  0.2505  0.2510 

H.  Nissenson,  who  employed  the  preceding  method  for 
determining  the  copper  and  lead  in  copper  matte,  gives  the 
following  directions  for  carrying  out  the  analysis : 

1  g  copper  matte  is  dissolved  in  30  cc  nitric  acid  (sp.  g. 
1.4)  and  the  solution  is  diluted  to  180  cc.  The  electrolysis 
is  so  conducted  that  the  lead  is  precipitated  on  the  dish,  a 
perforated  platinum  plate  which  serves  as  cathode  receiving 
the  copper.  The  electrolysis  is  started  at  ordinary  tempera- 
tures with  a  current  density  of  0.5  ampere,  which  at  the  end 
of  an  hour  is  increased  to  1.5-2  amperes.  Both  metals  are 
completely  precipitated  in  6-7  hours. 

For  technical  analyses,  where  the  determination  is  con- 
ducted from  nitric  acid  solutions,  the  presence  of  small 
quantities  of  silver  and  bismuth  may  be  neglected.  Where 
lead  is  precipitated  from  nitric  acid  solutions  containing 
arsenic,  selenium,  or  manganese,  even  in  very  small  quanti- 
ties, the  results  are  not  accurate. 

Copper — Silver. 
LITERATUEE  I 

Freudenberg,  Zeit.  f.  phys.  Chem.,  12,  97. 

Smith  and  Wallace,  Zeit.  f.  Elektrochemie,  5,  312. 

Heidenreich,  Ber.  deutsch.  chem.  Ges.,  28,  1585. 


216         QUANTITATIVE   ANALYSIS   BY   ELECTROLYSIS. 

Freudenberg  employs  a  solution  containing  a  few  cubic 
centimeters  of  nitric  acid  (sp.  g.  1.2)  for  the  separation  of  the 
silver,  which  is  quantitatively  precipitated  at  a  tension  of 
1.3-1.4:  volts.  The  copper  remains  in  solution  and  is  first 
decomposed  at  a  higher  tension  (2-3  volts). 

According  to  E.  Smith,  these  two  metals  may  be  sepa- 
rated from  a  solution  of  the  double  cyanides.  4. 5  g  of  potas- 
sium cyanide  are  added  to  a  solution  of  about  0.4  g  of  the 
mixed  metals.  The  solution  is  diluted  to  about  120  cc  and 
electrolyzed.  If  the  solution  be  warmed  to  65—75°,  the  pre- 
cipitation of  the  silver  is  greatly  hastened.  M.  Heidenreich 
tested  this  method  in  the  Aachen  laboratory  and  determined 
the  following  conditions  of  experiment. 

EXPERIMENT. 

Used  silver  nitrate  containing  63.42$  silver,  and  copper 
sulphate.  About  0.7  g  of  copper  sulphate  was  added. 


Taken 
gAgN03.    gKCN. 

0.2379        2 

Current 
Density, 
Amperes. 

0.07-0.03 

Electrode 
Tension, 
Volts. 

1.0-1.2 

Time,                               Found 
hr.        m.                             %  Ag. 

8        —                      63.34 

^0.2303 

2 

0.04 

1.0-1.28 

8 

63.43 

0  3099 

2 

0.03 

1.0-1.39 

6 

30                       63.40 

0.3327 

2 

0.09 

1.2-1,3 

4 

—  warmed        63.27 

0.6037 

6 

0.19-0.08 

1.2-1.3 

6 

—                       U3.33 

Copper — Mercury. 
LITER  AT  ORE  : 

Smith,  Journ.  of  Anal.  Chem.,  3,  254 ;  5,  489. 

Am.  Chem.  Journ.,  11,  104,  264. 
Freudenberg,  Zeit.  f.  phys.  Chem.,  12,  113. 

According  to  E.  Smith,  the  separation  may  be  conducted 
from  a  solution  of  the  double  cyanides.      The  temperature 


CADMIUM.  217 

should  be  about  65°.  With  the  ordinary  conditions  of  con- 
centration, about  2  g  potassium  cyanide  are  added,  and  the 
solution  is  electrolyzed  with  a  current  of  ND100  =  0.06-0.08 
ampere.  The  decomposition  requires  about  4  hours  for  every 
0.2  g  of  the  combined  metals.  The  copper  remains  in  solu- 
tion, the  mercury  being  deposited. 

Freudenberg  found  that  at  a  tension  of  2.5  volts  the 
mercury,  in  the  presence  of  2-4  g  potassium  cyanide,  sepa- 
rates brilliantly  white  and  completely  free  from  copper. 

Copper — Arsenic. 
LITERATURE  : 

Freudenberg,  Zeit.  f.  phys.  Chem.,  12,  97. 
Schmucker,  Zeit.  f.  anorg.  Chem.,  5,  199. 

Although  formerly  it  was  necessary  to  remove  the  arsenic 
before  precipitating  the  copper,  Freudenberg  has  shown  that 
a  separation  may  be  satisfactorily  conducted  from  a  sulphuric 
acid  solution  (10-20  cc  dilute  sulphuric  acid)  if  the  tension  is 
not  allowed  to  exceed  1.9  volts.  It  is  immaterial  whether 
the  arsenic  is  added  in  the  form  of  trioxide  or  pentoxide.  A 
second  method  of  the  same  author  is  the  following:  Am- 
monia is  added  to  the  solution  containing  the  metals  in  the 
form  of  higher  oxides,  until  there  is  an  excess  of  about  30  cc 
of  a  10#  ammonia  solution.  The  electrolysis  is  conducted 
with  a  current  tension  of  1.9  volts,  and  is  continued  until  the 
solution  is  completely  decolorized,  requiring  generally  6-8 
hours. 

This  method  is  not  suitable  for  the  separation  of  copper 
and  antimony. 

CADMIUM. 
Cadmium — Lead. 

This  process  is  the  same  as  the  separation  of  lead  from 
copper.  The  lead  is  separated  as  peroxide  from  a  nitric  acid 


218         QUANTITATIVE  ANALYSIS   BY   ELECTROLYSIS. 

solution.  To  determine  the  cadmium  in  the  solution  from 
which  the  lead  has  been  removed,  the  nitric  acid  is  evapo- 
rated off  on  the  water-bath,  the  cadmium  converted  into  sul- 
phate, and  treated  according  to  the  directions  on  page  164. 

Cadmium — Mercury. 
LITEEATUEE  I 

Freudenberg,  Zeit.  f.  phys.  Chem.,  12,  97. 

According  to  Freudenberg,  the  separation  proceeds  best 
from  a  solution  of  the  salts  of  both  metals,  containing  0.5-1 
g  potassium  cyanide.  With  a  tension  of  1.8-1.9  volts 
mercury  only  is  precipitated.  After  the  separation  of  the 
mercury,  the  cadmium  is  precipitated  from  the  solution  by  a 
current  of  higher  tension. 

LEAD. 

Lead— Silver. 

This  separation  is  conducted  like  that  of  lead  from  copper 
(see  page  213).  To  determine  the  silver,  the  solution  is 
evaporated  down  on  the  water-bath,  and  the  silver  is  precipi- 
tated according  to  the  directions  given  on  page  173. 

Lead — Mercury. 
LITEEATUEE  I 

Smith  and  Moyer,  Zeit.  f.  anorg.  Chem.,  4,  267. 
Heidenreich,  Ber.  deutsch.  chem.  Ges.,  28,  1585. 

The  method  corresponds  to  that  used  for  the  separation 
of  copper  from  lead.  Smith  and  Moyer  attempt  to  deter- 
mine the  lead  and  mercury  at  the  same  time.  They  add 
5  cc  nitric  acid  (sp.  g.  1.3)  to  the  solution  of  the  two  metals, 
and  dilute  the  liquid  to  180  cc.  The  electrolysis  is  con- 


LEAD.  219 

ducted  witli  a  current  of  1.7  cc  of  oxy  hydrogen   gas  per 
minute. 

Heidenreich  determined  the  conditions  of  experiment  for 
the  preceding  method,  and  found  that  20-30  cc  nitric  acid 
(sp.  g.  1.3-1.4)  must  be  present  for  every  120  cc  of  the 
solution  to  be  electrolyzed,  since  otherwise  the  lead  peroxide 
scales  off  and  cannot  be  accurately  determined.  A  current 
of  ND100  =  0.2-0. 5  ampere  may  be  used.  The  fact  that  greater 
quantities  of  lead  could  not  be  precipitated  in  an  adherent 
form  was  due  to  the  condition  of  the  surface  of  the  platinum 
disk  which  was  used  as  anode. 

Lead — Antimony. 
LITERATURE  I 

Neumann  and  Nissenson,  Chemiker  Zeitung,  1895,  No.  49. 

For  the  electrolytic  determination  of  both  metals  in  alloys 
(stereotype-metal,  type-metal),  Neumann  and  Nissenson  rec- 
ommend that  2.5  g  of  the  alloy  be  dissolved  by  warming 
with  a  mixture  of  10  g  tartaric  acid,  4  cc  nitric  acid  (sp.g.  1.4), 
and  15  cc  water.  4  cc  cone,  sulphuric  acid  are  then  added, 
the  solution  is  diluted  with  water,  allowed  to  cool,  and  filled 
up  to  exactly  one  quarter  liter.  If  the  liquid  is  now  filtered 
off  from  the  separated  lead  sulphate,  it  will  contain  all  of  the 
antimony.  50  cc  of  this  filtrate  are  made  strongly  alkaline 
with  sodium  hydroxide,  50  cc  of  a  saturated  solution  of  sodium 
monosulphide  are  added,  the  solution  is  filtered  immediately, 
washed  from  the  precipitate,  and  electrolyzed  according  to  the 
method  given  on  page  180. 

For  the  determination  of  the  lead,  the  lead  sulphate  is 
treated  as  in  the  separation  of  lead  from  copper  (page  213). 


220        QUANTITATIVE  ANALYSIS   BY  ELECTROLYSIS. 


SILVER. 

Silver — Antimony. 
LITERATUEE  I 

Freudenberg,  Zeit.  f .  phys.  Chem. ,  12,  97. 

If  the  antimony  is  present  as  pentoxide,  the  separation 
may  be  carried  out  from  an  ammoniacal  solution  to  which 
several  grams  of  ammonium  sulphate  have  been  added. 

The  silver  is  precipitated  at  a  tension  of  1.7-1.8  volts. 

Silver — Arsenic. 

LITERATUEE  I 

Freudenberg,  Zeit.  f.  phys.  Chem.,  12,  97. 

According  to  Freudenberg,  this  separation  is  conducted  in 
the  same  manner  as  the  separation  of  silver  from  antimony. 

MERCURY. 
Mercury — Antimony. 
LITEEATUEE  I 

Freudenberg,  Zeit.  f.  phys.  Chem.,  12,  97. 

The  antimony  must  be  added  in  the  form  of  a  pentavalent 
salt,  since  a  reduction  of  the  mercuric  salt  present  would 
otherwise  occur.  A  mixture  of  the  chlorides  of  the  two 
metals  is  brought  into  solution  by  the  use  of  0.5-1  g  tartaric 
acid.  The  solution  is  diluted  with  water,  made  neutral  with 
ammonia,  and  then  about  20  cc  of  a  10#  solution  of  ammonia 
are  added  until  the  solution  is  perfectly  clear.  The  electrolysis 
is  conducted  at  a  tension  of  1.6-1.7  volts.  After  the  mercury 


ANTIMONY.  221 

is  deposited,  the  solution  is  made  acid  and  hydrogen  sulphide 
is  passed  in.  The  antimony  sulphide  may  be  either  directly 
determined,  i.e.,  weighed,  or  determined  by  electrolysis  (see 
page  179). 

Mercury— Arsenic. 

LITERATURE  I 

Freudenberg,  Zeit.  f.  phys.  Chem.,  12,  97. 

According  to  Freudenberg,  the  separation  is  conducted 
from  a  nitric  acid  solution  (see  page  175)  from  which  the 
mercury  is  precipitated  at  a  tension  of  1.7-1.8  volts. 

ANTIMONY. 
Antimony — Tin. 
LITERATURE  : 

Classen,  Ber.  deutsch.  chem.  Ges.,  17,  2245  ;  18,  1110  ;  28,  2060. 

The  separation  of  antimony  from  tin  by  the  ordinary 
gravimetric  methods,  which,  as  is  well  known,  is  difficult,  and 
gives  uncertain  results,  may  be  accomplished  by  electrolysis 
with  ease  and  accuracy.  Antimony  may  be  completely  pre- 
cipitated, in  the  presence  of  tin,  from  a  concentrated  solution 
of  sodium  sulphide,  to  which  is  added  a  certain  amount  of 
sodium  hydroxide. 

The  crystallized  sodium  monosulphide  of  commerce,  aside 
from  the  fact  that  its  purity  is  otherwise  uncertain,  is  not 
pure  monosulphide,  but  is  a  mixture  of  several  sulphides  with 
varying  amounts  of  sodium  hydroxide.  This  explains  the 
large  amount  of  alumina  which  it  always  contains.  If,  there- 
fore, commercial  sodium  sulphide  is  to  be  used,  it  must  first 
be  dissolved,  and  the  solution,  with  exclusion  of  air,  com- 
pletely saturated  with  pure  hydrogen  sulphide  gas.  It  is  then 
filtered  from  the  precipitated  impurities,  and  evaporated  in  a 


222        QUANTITATIVE  ANALYSIS   BY   ELECTROLYSIS. 

large  platinum  or  porcelain  dish.  The  further  treatment  is 
given  in  full  in  the  chapter  on  reagents.  As  the  condition 
of  the  sodium  sulphide  solution  is  of  great  importance  to  the 
success  of  the  process,  it  is  preferable  to  prepare  the  solution 
as  directed  in  the  chapter  referred  to. 

The  process  of  separation  is  as  follows :  A  mixture  of  the 
pure  sulphides,*  or  the  residue  obtained  by  evaporating  a 
solution  of  the  two  metals,  is  treated  in  a  platinum  dish  with 
about  80  cc  of  a  sodium  sulphide  solution  saturated  at  ordinary 
.temperatures,  and  enough  concentrated  solution  of  pure 
sodium  hydroxide f  to  furnish  1-2  g  NaOH.  If  solution  does 
not  take  place  at  once,  it  is  hastened  by  heating  over  a  low 
Aflame,  the  watch-glass  covering  the  dish  is  rinsed  with  10-15 
cc  water,  and  the  solution  is  allowed  to  cool  thoroughly.  It 
is  then  submitted  to  electrolysis. 

When  weak  currents  (ND100  —  0.2  amp.)  are  employed, 
the  separation  of  the  antimony  requires  about  14  hours,  so  that 
the  electrolysis  must  be  continued  through  the  night.  Ex- 
periments recently  undertaken  by  the  author  have  shown 
that,  for  the  precipitation  of  antimony  in  the  presence  of  tin, 
the  solution  may  be  warmed  to  50-60°  and  a  current  density 
of  ND100  —  0.5  ampere  employed.  It  is  thus  possible  to 
complete  the  precipitation  within  2  hours. 

When  the  action  begins,  the  whole  surface  of  the  dish, 
which  is  in  contact  with  the  solution,  becomes  quickly  covered 
with  a  dark  coating  of  antimony,  which  soon  takes  on  a 
brilliant  metallic  appearance. 

*  The  solution  of  a  mixture  of  the  metallic  sulphides  and  sulphur  in 
sodium  sulphide  is  to  be  treated  like  a  solution  of  polysulphides  (see 
further  on). 

f  The  sodium  hydroxide  used  must  be  absolutely  pure,  and  must  show 
no  cloudiness  when  warmed  with  sodium  sulphide.  Otherwise  the  results 
obtained  for  the  antimony  will  be  too  high,  owing  to  the  inclusion  of  the 
precipitate. 


ANTIMONY.  223 

In  the  earlier  part  of  the  process,  the  entire  solution  ap- 
pears to  be  filled  with  small  gas-bubbles  which  rise  slowly, 
break  at  the  surface,  arid  cover  the  watch-glass  with  minute 
portions  of  the  solution.  In  the  course  of  two  hours  the  dis- 
engagement of  gas  is  ended,  and  the  solution  is  completely 
clear.  To  avoid  loss,  it  is  best,  at  this  time,  to  wash  re- 
peatedly the  under  surface  of  the  watch-glass  with  a  drop  of 
water  which  is  finally  allowed  to  run  down  the  positive  elec- 
trode. When  the  reduction  is  completed,  the  antimony  is 
washed  without  interrupting  the  current,  and  is  treated  accord- 
ing to  the  directions  already  given  (p.  180). 

As  tin  cannot  be  reduced  from  a  sodium  sulphide  solu- 
tion (as  stated  on  p.  184),  but  can  be  completely  precipitated 
from  solution  in  ammonium  sulphide,  the  sodium  sulphide, 
after  the  separation  of  antimony,  must  be  converted  into 
ammonium  sulphide  according  to  the  directions  given  on 
p.  187. 

If  the  two  metals  are  to  be  determined  in  the  yellow 
solution  of  polysulphides  of  the  alkalies,  the  solution  is 
decolorized  with  ammoniacal  hydrogen  peroxide  (see  Anti- 
mony, p.  181),  and  evaporated  nearly  to  dryness;  about  80 
cc  sodium  sulphide  solution  and  the  necessary  amount  of 
sodium  hydroxide  are  then  added,  and  the  process  carried  on 
as  above  directed. 

In  the  following  experiment,  antimony  was  precipitated 
from  both  warm  and  cold  solutions  containing  tin. 

EXPERIMENT. 

Used  about  1  g  antimony  potassium  tartrate,  an  equal 
quantity  of  NH4CLSnCl4,  80  cc  sodium  sulphide  solution, 
and  about  2  g  sodium  hydroxide. 


Current 
Density, 
Amperes. 

Electrode 
Tension, 
Volts. 

Temp. 

hrs? 

1.5-1.45 

0.9-0.8 

57-67° 

2 

1.5-1.6 

0.8-0.9 

58-60 

2 

0.4-0.2 

0.7-0.55 

30-24 

15 

224        QUANTITATIVE   ANALYSIS   BY   ELECTROLYSIS. 

Found  Taken 

Sb,  Sb, 

g.  g. 

0.3790  0.3780 

0.3787  0.3780 

0.3775  0.3780 

The  antimony  precipitate  appeared  gray  and  shiny,  and 
contained  no  tin. 

Antimony — Arsenic. 
LITEKATUKE  I  • 

Classen  and  Ludwig,  Ber.  deutsch.  chem,  Ges,,  19,  323. 

In  an  alkaline  solution,  arsenions  acid  is  oxidized  to 
arsenic  acid  by  the  galvanic  current.  If,  however,  a  solution 
containing  antimony  and  arsenious  acid  is  clectrolyzed,  a 
mixture  of  antimony  with  arsenic  is  deposited.  The  action 
is  different  if  the  arsenic  is  present  in  the  solution  as  arsenic 
acid ;  in  the  presence  of  a  free  alkali,  the  antimony  alone  is 
deposited  from  a  concentrated  sodium  sulphide  solution.  The 
arsenic,  therefore,  if  present  as  arsenious  acid,  must  be  oxi- 
dized to  arsenic  acid  before  the  metals  can  be  separated.  It 
is  heated  with  concentrated  nitric  acid  or  aqua  regia,  the  acid 
completely  removed  by  evaporation  on  the  water-bath,  the 
residue  treated  with  80  cc  of  a  cold  saturated  sodium  sul- 
phide solution,  a  concentrated  solution  of  sodium  hydroxide 
(containing  about  1 — 2  g  IsaOH)  added,  and  the  solution 
electrolyzed.  The  separation  is  conducted  precisely  like  that 
of  antimony  from  tin. 

The  electrolysis  may  be  conducted  either  warm  or  at  ordi- 
nary temperatures.  If  antimony  and  arsenic  are  to  be  deter- 
mined in  a  solution  of  polysulphides  of  the  alkalies,  the  solu- 
tion is  treated  as  described  on  p.  181.  To  determine  arsenic, 
the  antimony-free  solution  is  acidified  with  dilute  sulphuric 
acid,  heated  in  the  water-bath  to  remove  hydrogen  sulphide. 


ANTIMONY.  225 

filtered,  and  the  precipitate  dissolved  in  hydrochloric  acid 
with  the  addition  of  potassium  chlorate.  This  solution  is 
treated  with  ammonia  in  excess,  and  the  arsenic  acid  precip- 
itated as  magnesium  ammonium  arsenate  with  magnesium 
mixture. 

The  precipitate  may  be  dried,  at  110°,  on  a  weighed  filter, 
and  weighed,  or  converted  into  magnesium  pyro-arsenate  by 
careful  ignition  in  a  porcelain  crucible. 

EXPEKIMENT. 

Used  about  1  g  of  antimony  potassium  tartrate,  1  g 
sodium  arsenate,  80  cc  sodium  sulphide  solution,  and  2.5 
g  sodium  hydroxide. 


Current 
Density, 
Amperes. 

Electrode 
Tension, 
Volts. 

Temp. 

Time, 
hr.   m. 

Found 
Sb, 
g. 

Taken 
Sb, 
g. 

1.55-1.5 

1.75-1.1 

54-57° 

3     30 

0.3778 

0.3773 

1.60-1.5 

2.10-1.45 

25-38 

6     — 

0.3770 

0.3773 

0.5  -0.4 

1,75-0.8 

21-24 

overnight 

0.3770 

0.3770 

Antimony — Tin — Arsenic . 
LITERATURE  : 

Classen,  Ber.  deutsch.  chem.  Ges.,  17,  2245;  18,  1110;  28,  2060. 
Classen  and  Ludwig,  ibid.,  19,  323. 

If  arsenic  is  present  as  arsenic  acid,  antimony  alone  is 
precipitated  from  a  concentrated  alkaline  solution  of  the  three 
metals  in  sodium  sulphide ;  tin  and  arsenic  remain  in  solution. 
The  arsenic  is  converted  into  arsenic  acid,  and  the  antimony 
precipitated,  exactly  as  heretofore  described. 

For  the  separation  of  tin  from  arsenic,  the  solution  poured 
off  from  the  antimony  is  treated  with  dilute  sulphuric  or 
hydrochloric  acid  to  decompose  the  sulpho-salts,  the  mixture 
of  arsenic  and  tin  sulphides  and  sulphur  is  filtered  off  and 
oxidized  with  hydrochloric  acid  and  potassium  chlorate,  and 


226         QUANTITATIVE  ANALYSIS   BY    ELECTROLYSIS. 

the  arsenic  separated  as  described  below.  To  determine  the 
tin,  the  solution  freed  from  arsenic  is  saturated  with  hy- 
drogen sulphide,  filtered,  and  the  tin  sulphide  dissolved  in 
ammonium  sulphide.  The  tin  is  determined  electrolytically 
as  directed  p.  186. 

In  the  analysis  of  a  substance  which  contains  arsenic, 
antimony,  and  tin,  the  arsenic  may  also  be  first  eliminated 
according  to  the  method  of  E.  Fischer-Hufschmidt  simplified 
by  R.  Ludwig  and  the  author,*  and  antimony  and  tin  sepa- 
rated in  the  arsenic-free  solution. 

If  the  sulphides  of  the  metals  are  to  be  separated,  they 
are  oxidized  with  concentrated  hydrochloric  acid  and  potas- 
sium chlorate,  and  the  acid  evaporated  on  the  water-bath. 
The  residue  is  washed  with  fuming  hydrochloric  acid  into 
a  flask  of  500-600  cc  capacity, f  treated  with  20-25  cc  of  a 
saturated  solution  of  ferrous  chloride,  or,  better  with  about 
25gof  ammonium  ferrous  sulphate  [FeSO4.(NH4)2SO4.6H2O], 
and  fuming  hydrochloric  acid  added  till  the  volume  is  150 
to  200  cc.  A  strong  current  of  hydrochloric  acid  gas  is  now 
passed  into  the  solution,  and  kept  up  for  at  least  half  an  hour 
after  the  solution  seems  fully  saturated.  Then  the  solution 
is  reduced  to  about  50  cc  by  distilling  off  the  liquid,  without 
a  condenser,  in  a  stream  of  hydrogen  chloride  gas.  A  flask 
of  about  1  liter  capacity,  containing  400-500  cc  water,  is 
used  as  a  receiver.  If  the  flask  is  well  cooled  during  the 
distillation,  not  a  trace  of  arsenic  passes  over  into  a  second 
receiver,  even  when  as  much  as  0.5  g,  reckoned  as  As2O3,  is 
present. 

The  arsenic  in  the  distillate  may  either  be  saturated  with 
sodium  carbonate  and  titrated  with  iodine  solution,  or  pre- 

*  Ber.  d.  ch.  Ges.,  18,  1110. 

|  A  convenient  apparatus  is  illustrated  in  the  author's  "  Handbuch  der 
Quantitative  Analyse,"  4th  edition,  p.  78. 


ANTIMONY.  227 

cipitated  as  As3S3  with  hydrogen  sulphide,  and  determined 
as  such  on  a  weighed  filter,  or  the  arsenic  calculated  from  the 
amount  of  sulphur  in  the  precipitate.  The  process,  in  the 
latter  case,  is  as  follows :  The  distillate  is  mixed  with  twice 
its  volume  of  water,  air  expelled  by  a  strong  current  of 
carbon  dioxide,  and  the  arsenic  precipitated  by  passing  in 
pure  hydrogen  sulphide  gas.  The  excess  of  hydrogen  sul- 
phide is  removed  by  passing  a  strong  current  of  carbon 
dioxide  till  lead  acetate  paper  is  not  colored  by  the  escaping 
gases.  The  arsenic  sulphide  is  allowed  to  subside,  and  the 
clear  solution  siphoned  off.  The  remaining  strongly  acid 
solution  is  saturated  with  ammonia,  which  dissolves  the 
arsenic  sulphide ;  the  solution  is  then  boiled  with  an  excess 
of  hydrogen  peroxide  free  from  sulphuric  acid.  The  solution 
is  acidified  with  hydrochloric  acid,  and  the  sulphuric  acid 
produced  by  the  action  of  the  hydrogen  peroxide  determined 
as  barium  sulphate  in  the  usual  way  (Classen). 

To  determine  the  antimony  and  tin,  the  strong  acid  solu- 
tion in  the  flask,  which  contains  the  iron,  is  diluted  with 
three  times  its  volume  of  water.  Antimony  and  tin  are  pre- 
cipitated with  hydrogen  sulphide.  After  the  precipitate  has 
subsided,  the  clear  solution  is  poured  on  a  filter,  the  pre- 
cipitate washed  several  times  by  decantation,  and  afterwards, 
on  the  filter,  with  hot  water,  till  free  from  hydrochloric  acid. 
Portions  of  the  sulphides  often  adhere  to  the  walls  of  the  flask 
in  which  the  precipitation  took  place.  These  are  washed  out 
with  concentrated  sodium  sulphide  solution,  and  the  solution 
is  poured  on  the  filter  containing  the  sulphides.  The  filtrate 
is  collected  in  a  weighed  platinum  dish.  The  filter,  on  which 
some  iron  sulphide  always  remains  after  the  solution  of  the 
antimony  and  tin  sulphides,  is  washed  with  sodium  sulphide 
solution,  the  necessary  amount  of  sodium  hydroxide  is  added 


228         QUANTITATIVE   ANALYSIS   BY   ELECTROLYSIS. 

to  the  filtrate,  and  the  antimony  and  tin  are  separated  electro- 
lytically  as  already  directed. 

TIN— PHOSPHORIC  ACID. 

In  the  determination  of  metals,  in  the  presence  of  phos- 
phoric acid,  the  latter  is  often  removed  as  tin  phosphate. 
The  phosphoric  acid  is  then  usually  determined  in  a  separate 
portion,  as  its  determination  in  the  tin  precipitate  is  too 
difficult  and  slow  a  process.  The  precipitate  of  tin  oxide 
and  tin  phosphate  may,  however,  be  dissolved  by  digesting 
with  ammonium  sulphide,  the  solution  diluted,  the  tin  pre- 
cipitated by  electrolysis,  and  the  phosphoric  acid  determined 
as  usual. 

PLATINUM— IRIDIUM. 

As  stated  on  page  182,  platinum  can  be  separated  from  a 
hydrochloric  acid  solution  by  a  current  of  ]STD100  =  0.05  am- 
pere and  1.2  volts. 

This  property  of  platinum  may  be  used  for  separating  it 
from  iridium,  which  under  similar  conditions  remains  in  solu- 
tion. 

The  platinum  is  deposited  free  from  iridium.  (Classen.) 

SEPARATION  OF  GOLD  FROM  OTHER  METALS. 
LITERATURE  : 

Smith  and  Muhr,  Ber.  deutsch.  chem.  Ges.,  23,  2175. 
Smith  and  Wallace,  ibid.,  25,  779  ; 

Journ.  of  Anal.  Chem.,  1892,  p.  87. 

As  has  already  been  frequently  stated,  Edgar  F.  Smith 
has  made  an  exhaustive  study  of  the  action  of  the  galvanic 
current  on  the  cyanides  of  the  metals,  and  has  applied  this 


SODIUM— AMMONIA.  229 

method  to  the  separation  of  gold  from  palladium,  copper, 
nickel,  zinc,  and  platinum. 

The  same  conditions  may  also  be  employed  for  the  separa- 
tion of  silver  from  platinum  and  mercury  from  platinum. 

Smith  gives  but  incompletely  the  conditions  of  experi- 
ment necessary  for  conducting  these  operations,  and  therefore 
a  consideration  of  them  in  detail  will  be  omitted. 

POTASSIUM— SODIUM. 

The  ordinary  method  of  determining  potassium  and  so- 
dium in  the  same  solution  is  to  weigh  the  mixed  chlorides, 
and  the  potassium  as  platinchloride ;  the  sodium  is  thus  deter- 
mined by  difference.  The  errors  of  the  work,  therefore,  all 
fall  on  the  sodium.  The  potassium  may  be  determined,  as 
already  directed  (p.  188),  by  precipitating  as  potassium  platin- 
chloride, and  determining  the  platinum  in  the  latter  by  elec- 
trolysis. To  determine  the  sodium  directly,  the  filtrate  from 
the  potassium  platinchloride  is  evaporated  on  the  water-bath 
to  remove  alcohol,  the  residue  dissolved  in  water  with  the 
addition  of  a  little  hydrochloric  acid,  and  the  platinum  re- 
moved by  electrolysis.  The  sodium  chloride  in  the  solution 
poured  off  from  the  platinum  is  determined  by  evaporating  to 
dry  ness,  and  weighing  the  residue. 

SODIUM— AMMONIUM. 

The  direct  determination  of  both  is  accomplished  as  with 
potassium  and  sodium ;  the  ammonium  is  precipitated  as  ammo- 
nium platinchloride,  and  the  process  conducted  as  described 
above. 


APPENDIX. 


SOME  APPLIED   EXAMPLES   OF  ELECTBO- 
CHEMICAL  ANALYSIS.* 

BRASS. 
Alloy  of  Copper  and  Zinc  (Lead,  Tin,  Iron). 

For  the  separation  of  the  copper  from  the  other  metals,  it 
is  necessary  to  precipitate  it  from  an  acid  solution.  A  nitric 
or  sulphuric  acid  solution  may  be  used.  The  employment  of 
a  solution  containing  free  nitric  acid  has  the  disadvantage 
that  if  the  action  of  the  current  is  continued  for  too  long  a 
period  after  all  the  copper  has  been  precipitated,  the  nitric 
acid  is  reduced  to  ammonia,  and  zinc  is  precipitated.  It  has 
the  further  disadvantage  that  enough  ammonia  is  often  formed 
to  prevent  the  complete  precipitation  of  the  zinc  by  sodium 
carbonate,  a  method  often  employed  in  practice.  The  pres- 
ence of  nitric  acid  or  a  nitrate  also  prevents  the  electrolytic 
separation  of  the  zinc.  If  this  acid  is  used,  therefore,  the 
solution,  after  removal  of  the  copper,  must  be  repeatedly 

*  The  applied  examples  of  electro- analysis  here  given  appeared  in  the 
third  German  and  second  English  editions  of  this  work,  but  are  not  con- 
tained in  the  fourth  German  edition.  Owing  to  the  practical  advantages  of 
these  schematic  outlines,  the  translators  have  thought  it  best  to  include 
them  in  the  present  edition,  and  have,  at  the  same  time,  made  such  altera- 
tions as  the  recent  advances  along  the  variouj^Uww^wJd^eem  to  justify. 

231 


232         QUANTITATIVE   ANALYSIS   BY   ELECTROLYSIS. 

evaporated  to  dryness  with  hydrochloric  acid  to  convert  the 
nitrates  into  chlorides. 

For  the  analysis  of  the  alloy,  0.1-0.2  g  is  dissolved  in  as 
little  dilute  nitric  acid  as  possible,  and  evaporated  to  dryness 
on  the  water-bath.  The  residue  is  then  treated  with  a  few 
cubic  centimeters  of  water,  and  20  cc  nitric  acid  (sp.  g.  =  1.27) 
are  added.  The  solution  is  now  diluted  to  about  100  cc,  and 
any  stannic  oxide  which  may  be  present  is  filtered  off  and  de- 
termined gravimetrically.  From  the  solution,  the  final  vol- 
ume of  which  should  be  120  cc,  the  copper  is  precipitated  by 
electrolysis  according  to  the  directions  given  on  page  156. 
The  current  is  continued  as  long  as  a  drop  of  the  solution 
gives  a  blue  color  with  ammonia. 

If  lead  is  present  in  the  alloy  it  may  be  determined  at  the 
same  time  as  the  copper,  since  it  separates  on  the  positive 
electrode  in  the  form  of  peroxide.  A  weighed  positive  elec- 
trode is  employed,  and  the  precipitated  peroxide  is  washed 
and  treated  according  to  the  directions  on  page  169.  The 
separated  lead  peroxide  and  copper  are  washed  without  inter- 
rupting the  current. 

The  zinc  is  best  determined  in  the  solution  by  adding 
about  5  cc  dilute  sulphuric  acid  and  evaporating  on  the  water- 
bath  until  no  odor  of  nitric  acid  can  be  detected.  The  residue 
is  dissolved  in  a  small  quantity  of  water,  and  a  slight  excess 
of  ammonia  added.  If  iron  is  present  it  will  be  precipitated 
as  hydroxide,  which  may  be  filtered  off  from  the  solution  and 
determined  gravimetrically.  Ammonium  oxalate  or  lactate  is 
now  added,  and  the  separation  of  the  zinc  conducted  under 
the  conditions  *  given  on  page  147. 

When  a  sulphuric  acid  solution  is  employed  for  the  sepa- 

*  The  same  electrode  upon  whicli  the  copper  has  been  precipitated  may 
be  used  for  receiving  the  zinc.  By  this  the  necessity  of  especially  prepar- 
ing a  copper-plated  electrode  is  avoided. 


APPENDIX.  233 

ration  of  the  copper,  it  is  best  to  first  dissolve  the  alloy  in 
dilute  nitric  acid  and  filter  off  any  stannic  oxide  as  before, 
xln  excess  of  sulphuric  is  then  added,  and  the  solution  is 
evaporated  until  all  nitric  acid  is  driven  off.  The  residue  is 
now  treated  with  water,  any  lead  which  is  present  being  then 
found  in  the  form  of  sulphate,  which  can  be  removed  by 
filtering  and  determined  gravimetrically.  The  solution  is 
diluted  to  115  cc,  5  cc  nitric  acid  (sp.  g.  ==  1.21)  are  added, 
and  the  precipitation  of  the  copper  conducted  under  the  con- 
ditions given  on  page  156.  After  the  copper  has  been  sep- 
arated, the  solution  is  evaporated  to  drive  off  nitric  acid,  and 
the  separation  of  the  zinc  is  carried  out  as  in  the  previous 
case. 

SILVER  COIN. 
Alloy  of  Copper  and  Silver. 

The  alloy  is  analyzed  by  dissolving  0.1-0.2  g  in  dilute 
nitric  acid,  evaporating  off  the  acid  on  the  water-bath,  dis- 
solving the  residue  in  water,  and  treating  the  solution  accord- 
ing to  the  directions  on  page  216. 

NICKEL    COIN. 
Alloy  of  Copper  and  Nickel. 

About  0.4  g  of  the  alloy,  best  in  the  form  of  small  cut- 
tings, is  dissolved  in  dilute  nitric  acid,  8  cc  of  dilute  sulphuric 
acid  (50  per  cent)  is  added,  and  the  solution  is  evaporated  on 
the  water-bath  until  all  nitric  acid  is  removed.  The  residue 
is  then  taken  up  in  150  cc  of  water,  and  electrolyzed  with  a 
current  of  ND100  =  1  ampere,  and  an  electrode  tension  of 
2.75-3  volts. 

After  the  removal  of  the  copper  the  solution  is  neutralized 
with  ammonia,  an  excess  of  40  cc  ammonia  (sp.  g.  0.96)  is 


234         QUANTITATIVE  ANALYSIS   BY    ELECTROLYSIS. 

added,  and  the  nickel  is  precipitated  by  a  current  of  OT3100  =• 
0.5-1.5  amperes,  and  a  tension  at  the  electrodes  of  2.8-3.3 
volts. 

GERMAN   SILVER. 
Alloy  of  Copper,  Zinc,  Nickel  (Tin,  Lead). 

For  the  analysis  of  this  alloy  about  0.3  g  of  the  metal  is 
dissolved  in  nitric  acid,  10  cc  concentrated  nitric  acid  added, 
the  solution  diluted  to  150  cc,  and  electrolyzed  at  ordinary 
temperatures  with  a  current  of  !ND100  =  0.5-1  ampere,  and 
an  electrode  tension  of  2.5-2.8  volts.  The  solution  from 
which  the  copper  has  been  removed  is  evaporated  to  dryness 
with  sufficient  sulphuric  acid  to  convert  the  nitrates  present 
into  sulphates,  and  the  residue  is  dissolved  in  water. 

To  the  solution  containing  the  zinc  and  nickel  5  g  potas- 
sium sodium  tartrate  is  added,  and  the  solution  is  made  alka- 
line with  sodium  hydroxide.  The  zinc  is  now  precipitated 
with  a  current  of  ND100  =  0.3-0.6  ampere,  and  an  electrode 
tension  of  2  volts.*  The  zinc  may  be  precipitated  on  the 
electrode  bearing  the  copper  precipitate.  In  this  operation 
oxide  of  nickel  may  separate  on  the  positive  electrode,  or  may 
form  in  the  solution  in  sufficient  quantities  to  slightly  discolor 
the  precipitated  zinc.  This  may  be  avoided  by  adding  to  the 
solution  a  small  quantity  of  potassium  iodide. 

The  solution,  containing  now  only  nickel,  is  acidified  with 
sulphuric  acid,  an  excess  of  ammonia  added,  and  the  nickel 
separated  according  to  the  directions  for  cobalt  given  on  page 
142.  Another  method  is  to  add  25  cc  ammonia  and  15-20  g 
ammonium  carbonate  directly  to  the  nickel  solution,  and  elec- 
trolyze  with  a  current  of  KT>100  =  0.8-1  ampere,  at  a  temper- 
ature of  50-60°. f 

*  Vortmann,  Monatsh.  f.  Chem.,  14,  536. 

f  Neumann,  Analytiscben  Elektrolyse,  Halle,  1897. 


APPENDIX.  235 

BRONZE. 
Alloy  of  Copper  and  Tin. 

The  alloy  in  a  finely  divided  form  is  treated  with  aqua 
regia,  and  the  solution  is  evaporated  to  dryness.  The  residue 
is  digested  with  a  concentrated  solution  of  sodium  sulphide, 
the  tin  being  dissolved.  The  copper  sulphide  which  remains 
after  filtering  is  washed  thoroughly  with  sodium  sulphide  and 
then  with  hydrogen  sulphide  solution,  dissolved  in  the  proper 
quantity  of  nitric  acid,  and  the  copper  precipitated  under  the 
conditions  given  on  page  156. 

The  solution  of  tin  in  sodium  sulphide  is  brought  to  a 
volume  of  about  150  cc,  25—30  g  ammonium  sulphate  is 
added,  and  the  solution  is  boiled  for  about  one  half  hour  to 
convert  the  sodium  sulphide  into  ammonium  sulphide  (see 
page  187).  The  solution  thus  obtained  is  treated  as  described 
on  page  186.  . 

Accurate  results  may  also  be  obtained*  by  treating 
0.2-0.4  g  of  the  alloy,  best  in  the  form  of  fine  turnings, 
with  6  cc  nitric  acid  (sp.  g.  =  1.5),  and  adding  3  cc  water. 
When  the  reaction  is  over,  the  solution  is  heated  to  boiling, 
diluted  with  15  cc  boiling  water,  and  the  stannic  oxide  filtered 
off.  To  the  solution  containing  the  copper,  5-10  cc  of  nitric 
acid  is  added,  and  the  copper  is  precipitated  as  directed  on 
page  156.  The  stannic  oxide  is  dissolved  in  ammonium  sul- 
phide and  determined  electrolytically  (page  186). 

PHOSPHOR-BRONZE. 
Alloy  of  Copper,  Tin,  Zinc,  and  Phosphorus. 

When  the  alloy  is  digested  with  concentrated  nitric  acid 
as  stated  under  Bronze,  a  precipitate  remains,  which  consists 

*  Neumann,  1.  c. 


236         QUANTITATIVE   ANALYSIS   BY   ELECTROLYSIS. 

of  a  mixture  of  tin  oxide  and  tin  phosphate,  with  small  quan- 
tities of  copper  oxide.  It  is  filtered  off,  washed  with  water 
containing  nitric  acid,  and  heated  with  a  concentrated  solution 
of  sodium  culphide.  The  residue  of  copper  sulphide  is  dis- 
solved in  nitric  acid,  and  added  to  the  principal  solution. 

The  tin  is  determined  by  converting  the  sodium  sulphide 
into  ammonium  sulphide,  and  electrolyzing  as  directed  p.  186. 
The  phosphoric  acid  is  determined  in  the  filtrate  in  the  usual 
manner. 

The  nitric  acid  solution  contains  the  copper  and  zinc. 
They  are  separated  according  to  directions  for  the  analysis  of 
brass  (p.  231). 


MANGANESE  PHOSPHOR-BRONZE. 

Alloy  of  Copper,  Tin,  Zinc,  Manganese,  and  Phosphorus. 

The  process  is  similar  to  that  given  for  Phosphor- Bronze ; 
the  manganese  remains  with  the  zinc,  and  is  finally  separated 
as  directed  p.  208. 

SOLDER. 

Alloy  of  Tin  and  Lead. 

About  0.4  g  of  the  alloy  in  the  form  of  small  pieces  is 
treated  with  6  cc  nitric  acid  (sp.  g.  —  1.5)  and  3  cc  water. 
When  the  reaction  is  completed  the  solution  is  heated  to  boil- 
ing, and  diluted  with  15  cc  hot  water,  the  precipitate  of  stan- 
nic oxide  allowed  to  settle,  filtered  oft',  and  washed  with  water 
containing  a  little  nitric  acid.  The  stannic  oxide  may  be 
determined  gravimetrically,  or  may  be  dissolved  in  ammonium 
sulphide  and  determined  by  electrolysis  according  to  the  direc- 
tions given  on  page  186.  The  lead  contained  in  the  filtrate 
may  be  determined  by  the  method  given  on  page  169. 


APPENDIX.  237 

WOOD'S  METAL. 
Alloy  of  Tin,  Lead,  Bismuth,  and  Cadmium. 

The  alloy  is  treated  similarly  to  solder,  the  tin  being  sepa- 
rated and  determined  in  the  same  manner.  Since  it  is  impos- 
sible to  separate  lead  and  bismuth  by  electrolysis,  it  is  necessary 
to  evaporate  the  solution  to  a  sirup  on  the  water-bath,  add 
water  and  repeat  the  operation  until  the  odor  of  nitric  acid 
can  be  no  longer  detected.  The  solution  is  then  treated  with 
dilute  ammonium  nitrate  solution,  and  the  basic  bismuth 
nitrate  is  filtered  oft'.*  A  sufficient  excess  of  nitric  acid  is 
added  to  the  filtrate,  and  the  lead  is  determined  by  electroly- 
sis. The  cadmium  is  precipitated  by  one  of  the  methods 
given  under  Cadmium. 

HARD  LEAD.     TYPE-METAL. 

Alloy  of  Lead  and  Antimony  (Copper). 

The  two  metals  may  be  separated,  either  by  oxidizing  with 
nitric  acid,  evaporating  to  dryness,  and  digesting  the  residue 
with  sodium  sulphide,  or  by  heating  the  finely  divided  alloy 
with  ten  times  its  weight  of  anhydrous  sodium  thiosulphate  in 
a  covered  porcelain  crucible,  over  a  very  low  fiame,  till  the 
mixture  is  sintered  together,  and  extracting  with  water.  In 
either  case,  lead  sulphide  remains  undissolved,  and  is  filtered 
off,  and  washed  first  with  sodium  sulphide,  and  then  with  hy- 
drogen sulphide,  solution.  It  may  be  determined  directly  as 
sulphide,  or  as  directed  p.  169. 

The  antimony  is  determined,  in  the  filtrate  from  lead  sul- 
phide, exactly  as  directed  p.  180. 

The  following  method  is  recommended  by  Neumannf : 
2.5  g  of  the  alloy  are  brought  into  a  250-cc  graduated  flask, 

*  Neumann,  Analytisclien  Elektrolyse,  Halle,  1897. 
f  Analytischen  Elektrolyse,  Halle,  1897. 


238         QUANTITATIVE  ANALYSIS   BY   ELECTKOLYSIS. 

10  g  tartaric  acid,  15  cc  water,  and  4  cc  strong  nitric  acid  are 
added,  and  solution  is  effected  by  warming.  To  the  clear 
solution  4  cc  of  concentrated  sulphuric  acid  is  added,  it  is  di- 
luted somewhat,  allowed  to  cool,  and  then  diluted  to  the  mark. 
50  cc  of  the  filtrate,  corresponding  to  0.5  g  of  the  substance, 
is  made  strongly  alkaline  with  sodium  hydroxide,  treated  with 
50  cc  saturated  sodium  sulphide  solution,  heated  to  boiling, 
and  immediately  filtered.  The  filtrate,  while  still  hot,  is 
electrolyzed  with  a  strong  current  according  to  the  directions 
given  on  page  181.  For  the  determination  of  the  copper 
which  is  present,  the  residue  remaining  after  treating  with 
sodium  sulphide  is  dissolved  in  nitric  acid,  the  solution  is  di- 
luted, and  the  copper  separated  as  given  on  page  156.  If  the 
percentage  of  lead  is  also  required,  0.5  g  of  the  alloy  may  be 
taken  and  the  precipitated  lead  sulphate  determined  gravi- 
metrically ;  it  is  more  satisfactory,  however,  to  treat  the  solu- 
tion of  the  metals  directly  with  sodium  hydroxide  and  sodium 
sulphide.  The  residue,  consisting  of  the  sulphides  of  lead 
and  copper,  is  then  dissolved  in  nitric  acid,  and  the  separation 
of  the  two  metals  is  conducted  under  the  conditions  given  on 
j)age  213. 

ALLOY   OF   ANTIMONY   AND   TIN. 

The  method  of  analysis  has  been  already  given  on  p.  121. 
The  alloy  is  oxidized  with  nitric  acid,  and  the  residue,  after 
evaporation,  dissolved  in  a  concentrated  solution  of  sodium 
sulphide,  sodium  hydroxide  added,  and  the  process  followed 
throughout  as  given  on  p.  122. 

ALLOY   OF   ANTIMONY   AND   ARSENIC. 

It  has  already  been  stated  (p.  224)  that  the  two  metals 
can  be  separated  under  conditions  similar  to  those  in  the 


APPENDIX.  239 

separation  of  antimony  from  tin  ;  the  method  requires  the 
arsenic  to  be  oxidized  to  arsenic  acid.  The  alloy  is  digested 
with  aqua  regia,  the  acid  removed  by  evaporation,  the  residue 
dissolved  in  concentrated  sodium  sulphide,  sodium  hydroxide 
added,  and  the  directions  given  on  p.  225  followed  throughout. 

ALLOY     OF    ANTIMONY,    TIN,    AND     ARSENIC. 

When  this  alloy  is  oxidized  with  aqua  regia,  and  a  solu- 
tion in  sodium  sulphide  prepared  as  above,  antimony  alone  is 
electrolytically  deposited  in  presence  of  tin.  The  method  is 
described  on  p.  225. 

SPATHIC    IRON    ORE. 

Constituents  :    Ferrous   Carbonate,  -with  Manganese,   Calcium, 
and  Magnesium  Carbonates   (Gangue). 

All  the  constituents  of  the  mineral  may  be  determined  in 
the  same  solution.  About  0.5  g  of  the  dry  mineral  is 
dissolved  in  a  porcelain  dish,  in  the  least  possible  amount  of 
hydrochloric  acid,  the  acid  removed  by  evaporation,  and  the 
residue  taken  up  with  water  to  which  a  little  hydrochloric 
acid  is  added.  If  insoluble  gangue  is  present,  this  is  filtered 
off,  washed  with  water,  and  weighed.  The  metals  are  con- 
verted into  oxalates  by  treatment  with  potassium  and  ammo- 
nium oxalate,  and  the  insoluble  residue  of  calcium  oxalate 
filtered  off,  and  washed  with  hot  water.  If  manganese  is  pres- 
ent, the  calcium  oxalate  always  carries  down  some  manganese 
oxalate.*  When  the  precipitate  is  ignited,  a  mixture  of  CaO 
and  MTi2O3  is  obtained.  It  is  weighed,  and  the  manganese  in 
it  determined  volumetrically.f 

The  iron  and  manganese  are  separated  as  directed  on  p.  195, 

*  Classen,  Zts.    anal.  Ch.,  16,  318. 

f  Classen,  Quant.  Anal.,  4th  ed.,  p.  128. 


240         QUANTITATIVE   ANALYSIS   BY    ELECTKOLYSIS. 

the  manganese  finally  precipitated  as  sulphide,  and  the  mag- 
nesium in  the  filtrate  as  magnesium  ammonium  phosphate. 
If  magnesium  is  absent,  the  manganese  is  determined  as 
mangano-manganic  oxide  or  sulphate  (p.  196). 

HEMATITE. 

Constituents:  Ferric  Oxide,  Manganic  Oxide  (Copper  Oxide,  Alumina, 
Lime.  Magnesia),  Phosphoric  Acid,  Sulphuric  Acid. 

The  iron,  manganese,  and  calcium  are  determined  a& 
above.  If  copper  is  present,  it  is  first  separated  from 
the  other  metals  by  submitting  the  double  oxalate  solution 
to  a  very  weak  current.  If,  in  addition  to  iron  (copper, 
if  present)  and  manganese,  phosphoric  and  sulphuric  acids 
are  to  be  determined,  the  metals  are  converted  into  double 
oxalates,  and  iron  and  manganese  completely  removed  (see 
separation  of  Iron  and  Manganese,  p.  195);  the  two  acids 
may  now  be  determined  in  the  solution  entirely  free  from 
manganese.  If  only  one  acid  is  to  be  determined,  the  whole 
filtrate  can  be  used ;  otherwise  it  is  diluted  to  a  known 
volume,  and  aliquot  portions  taken  for  analysis.  In  the 
determination  of  either  acid,  the  solution  is  first  acidified 
with  hydrochloric  acid,*  and  then  treated  either  with  barium 
chloride,  or  with  one-third  its  volume  of  ammonia,  and  mag- 
nesium mixture.  About  1  g  of  the  mineral  is  needed  for 
the  determination  of  sulphuric  and  phosphoric  acids. 

If  alumina,  as  well  as  phosphoric  acid,  is  present  in 
hematite  (its  presence  is  shown  by  a  white  turbidity  f  of 

*  If  the  acid  carbonates  produced  from  the  oxalates  are  not  decomposed, 
small  hard  crystals  of  acid  carbonates  are  precipitated  together  with  am- 
monium magnesium  phosphate.  These  crystals  are  difficultly  soluble  in 
ammonia,  and  may  make  the  results  too  high. 

t  A  turbidity  often  appears  when  the  solution  is  first  heated,  caused  by 
the  driving  off  of  ammonium  compounds. 


APPENDIX.  241 

aluminium  phosphate  and  hydroxide  in  the  solution  under- 
going electrolysis),  the  manganese  must  always  be  converted 
into  sulphide.  The  iron-free  solution  is  boiled  to  decompose 
hydrogen  ammonium  carbonate,  tartaric  acid  or  a  solution  of 
a  tar tr ate  added  till  the  precipitate  of  aluminium  hydroxide 
disappears,  and  the  weakly  ammoniacal  solution  precipitated 
hot  with  ammonium  sulphide. 

The  green  manganous  sulphide  is  determined  as  hereto- 
fore directed.  The  phosphoric  acid  may  be  determined  with 
magnesium  mixture,  in  the  filtrate  from  the  manganese  sul- 
phide. 

To  determine  sulphuric  acid  in  presence  of  alumina, 
iron  and  manganese  are  removed,  by  electrolysis,  from  a 
separate  portion,  the  solution  is  poured  off,  the  ammonium 
carbonate  decomposed  by  heat,  the  solution  acidified  with 
hydrochloric  acid,  and  the  sulphuric  acid  determined  with 
barium  chloride. 

Determination  of  Iron,  Manganese,  Copper,  Calcium,  Magnesium, 
Phosphoric  Acid,  and  Sulphuric  Acid. 

The  method  of  determining  iron,  manganese,  etc.,  in  the 
same  solution  has  already  been  given.  If  it  is  desired  to 
determine  magnesium  and  phosphoric  and  sulphuric  acids, 
in  the  filtrate  from  manganese  peroxide,  it  is  diluted  to  a 
known  volume,  magnesium  is  determined  in  an  aliquot  part 
with  ammonium  phosphate,  and  phosphoric  and  sulphuric 
acids  in  two  other  portions. 

LIMONITE. 

Constituents :  Ferric  Hydroxide,  together  with  Manganese  Oxide 
(Lime,  Magnesia),  Phosphoric  Acid,  Sulphuric  Acid,  Silica, 
and  Gangue. 

The  analysis  may  be  conducted  like  those  of  hematite 
and  spathic  iron  ;  but  care  must  be  taken,  at  the  outset,  to 


242        QUANTITATIVE  ANALYSIS    BY    ELECTROLYSIS. 

convert  the  silica  into  the  insoluble  modification  by  evapo- 
rating the  solution,  and  drying  the  residue. 

CLAY    IRON-ORE. 
Constituents  :    Iron   Oxide,  Alumina,  Manganese,  and  Water. 

The  mineral  is  digested  with  concentrated  hydrochloric 
acid  till  it  is  completely  decomposed,  the  insoluble  residue  is 
filtered  off,  the  filtrate  evaporated  to  remove  free  acid,  the 
residue  dissolved  in  water  with  a  few  drops  of  hydrochloric 
acid,  and  the  iron  separated  from  aluminium  and  manganese 
as  directed  pp.  194-198. 

BOG    IRON-ORE. 

Mixture  of  Ferric  Hydroxide  -with  Ferrous  and  Ferric  Silicates, 
Manganese,  Alumina,  Copper,  Calcium,  Magnesium,  Sulphuric 
Acid,  Phosphoric  Acid,  Arsenic  Acid,  Organic  Matter,  and 
Gangue. 

The  analysis  of  the  mineral  is  easily  understood  from  the 
foregoing. 

Arsenic  and  copper  are  best  determined  by  eliminating 
the  former  as  chloride,  as  directed  p.  226,  and  precipitating 
the  copper  with  hydrogen  sulphide  in  the  greatly  diluted 
residue  left  in  the  distillation  flask.  The  copper  sulphide  is 
dissolved  in  nitric  acid,  and  determined  electrolytically  as 
directed  p.  156. 

CHROME    IRON     ORE. 

Constituents :    Chromium    Oxide,    Ferrous    and    Ferric    Oxides, 
Alumina,  Manganese,   Calcium,  Silica. 

The  finely  powdered  mineral  is  fused  for  a  long  time  with 
sodium  carbonate  and  potassium  chlorate,  and  the  fused  mass 


APPENDIX. 


extracted  with  water.  The  residue  contains  oxides  of  iron., 
manganese,  calcium,  magnesium,  and  aluminium,  and  traces 
of  chromium  and  silica ;  the  solution,  chromic  acid,  silica, 
and  some  alumina  and  lime.  The  residue  is  dissolved  in 
hydrochloric  acid,  the  solution  evaporated  to  dryness  to 
separate  silica,  the  residue  treated  with  water  and  a  little 
hydrochloric  acid,  and  filtered.  The  metals  in  the  filtrate 
are  converted  into  double  oxalates.  If  manganese  is  present, 
the  precipitate  of  calcium  oxalate  must  be  treated  as  directed 
p.  239.  The  filtrate  from  the  calcium  oxalate,  which  contains 
iron,  manganese,  aluminium,  and  chromium,  is  treated  as 
directed  pp.  194—199.  The  aqueous  solution  from  the  fused 
mass  is  evaporated  to  separate  silica,  the  calcium  precipitated  as 
oxalate,  and  the  aluminium  and  chromic  acid  separated  accord- 
ing to  previous  directions. 

Edgar  F.  Smith  recommends  the  use  of  the  galvanic  cur- 
rent for  the  decomposition  of  chrome  iron  ore.  The  process, 
according  to  his  directions,  is  conducted  as  follows :  Thirty 
or  forty  gin.  potassium  hydroxide  are  heated  in  a  nickel 
crucible  until  the  mass  is  in  a  condition  of  quiet  fusion.  The 
chrome  iron  ore  for  decomposition  (about  0.5  gm.)  is  finely 
pulverized,  weighed  on  a  watch-glass,  and  gradually  added, 
with  the  help  of  a  camel's-hair  pencil,  to  the  crucible  contain- 
ing the  fused  alkali.  The  crucible  is  then  covered  with  a 
perforated  watch-glass  and  connected  with  the  anode  of  the 
battery  or  other  source  of  current.  The  kathode  employed  is 
a  thick  platinum  wire,  which  is  plunged  through  the  opening 
in  the  watch-glass  into  the  fused  mass.  To  regulate  the  current 
an  amperemeter  (p.  31)  is  inserted,  and  a  switch  is  also  placed 
in  the  circuit,  so  adjusted  as  readily  to  produce  the  reversal  of 
the  current,  which  is  necessary  toward  the  close  of  the  process. 
The  current  strength  must  not  exceed  1  ampere.  After  about 
30  minutes  the  current  is  reversed  by  the  switch,  so  that  the 


244         QUANTITATIVE   ANALYSIS    BY    ELECTROLYSIS. 

crucible  becomes  the  kathode,  and  the  platinum  wire  the  anode. 
The  object  of  this  reversal  is  to  oxidize  completely  the  last 
traces  of  the  mineral,  minute  portions  of  which  may  have  been 
protected  by  metallic  iron  which  had  been  deposited  by  the 
current.  After  the  current  has  acted  in  this  direction  for  10 
minutes,  the  decomposition  is  complete.  The  fused  mass 
of  course  contains  the  chromium  as  chromate. 

The  author  pursued  similar  researches  some  years  since, 
and  can  confirm  Smith's  results. 

PSILOMELANE. 

Constituents :  Manganous  Oxide,  Copper  Oxide,  Ferric 
Oxide,  Nickel  Oxide,  Cobalt  Oxide,  Alumina,  Lime, 
Potash,  Soda,  and  Lithia. 

Determination  of  Manganese,  Copper,  Iron,  Aluminium, 
Nickel,  Cobalt,  and  Calcium. 

A  weighed  portion  of  the  mineral  is  dissolved  in  hydro- 
chloric acid,  evaporated  to  dryness,  dissolved  in  water  with 
a^  few  drops  of  hydrochloric  acid,  converted  into  double 
oxalates,  calcium  oxalate  filtered  off,  and  the  calcium  and 
manganese  in  the  precipitate  determined  as  directed  p.  239. 
In  the  filtrate,  the  copper  is  first  determined  electrolyti- 
cally  (p.  155).  After  the  precipitation  of  the  copper  is 
complete,  the  solution,  which  contains  the  other  metals, 
is  decanted  from  the  copper  precipitate,  and  is  then  again 
submitted  to  electrolysis  for  the  precipitation  of  iron,  co- 
balt, nickel,  and  manganese,  the  latter  as  dioxide  at  the 
positive  electrode.  After  the  electrolysis  is  completed,  the 
solution  is  decanted  from  the  precipitated  metals,  and  the 
remaining  manganese  completely  precipitated,  according  to 
directions  given  on  p.  196.  If  only  the  weight  of  nickel 
and  cobalt  together  is  desired,  the  precipitate  containing  the 


APPENDIX.  245 

three  metals  is  dissolved  in  hydrochloric  acid,  and  the  iron 
determined  by  titration  with  potassium  permanganate  as 
directed  p.  191.  Otherwise  the  cobalt  and  nickel  must  first 
be  separated  from  the  iron.  The  precipitate  of  the  metals  is 
dissolved  in  hydrochloric  acid,  the  acid  removed  by  evapora- 
tion, the  residue  oxidized  with  hydrogen  peroxide  or  bromine 
water,  dissolved  in  water  with  a  few  drops  of  hydrochloric 
acid,  and  the  metals  converted  into  double  oxalates  by  addi- 
tion of  potassium  oxalate  in  slight  excess.  From  the  boiling 
solution,  which  should  have  a  volume  of  80-100  cc,  the 
cobalt  and  nickel  are  precipitated  as  oxalates  by  concen- 
trated acetic  acid.  A  great  excess  of  acetic  acid  must  be 
used,  and  the  solution,  after  the  filtrate  has  subsided,  must 
be  tested  with  the  reagent  for  a  further  precipitate.  The 
filtrate  from  the  cobalt  and  nickel  oxalates  contains  all  the 
iron  as  potassium  iron  oxalate.* 

The  precipitate  of  nickel  and  cobalt  oxalates  is  washed 
with  a  mixture  of  equal  parts  of  alcohol,  acetic  acid,  and 
water,  and,  after  drying  to  remove  acetic  acid  and  alcohol,  is 
dissolved  on  the  filter  with  hot  water  containing  potassium 
and  ammonium  oxalates.  The  solution  is  electrolyzed  as 
directed  p.  141.  The  sum  of  nickel  and  cobalt  is  determined, 
the  metals  dissolved  in  hydrochloric  acid,  evaporated  to  dry- 
ness,  the  residue  dissolved  in  a  few  drops  of  water,  potassium 
hydroxide  added  in  slight  excess,  and  the  resulting  precipi- 
tate dissolved  in  concentrated  acetic  acid.  The  cobalt  is 
precipitated  with  a  saturated  solution  of  potassium  nitrite 
acidified  with  acetic  acid.  The  precipitate,  after  standing 
twenty-four  hours,,  is  filtered  off,  washed  with  potassium 
nitrite,  and  dissolved  in  hydrochloric  acid,  the  solution  is 

*  Classen,  Zts.  anal.  Ch.,  18,  189 


246         QUANTITATIVE  ANALYSIS   BY    ELECTROLYSIS. 

evaporated  to  dryness,  and  the  residue  converted  into  the 
double  oxalate,  and  electrolyzed.  The  nickel  is  determined 
by  difference.  The  nickel  may  also  be  determined,  instead 
of,  or  in  addition  to,  the  cobalt,  by  precipitating  nickel  with 
potassium  hydroxide,  in  the  filtrate  from  the  cobalt  potas- 
sium nitrite,  filtering,  dissolving  in  hydrochloric  acid,  and 
separating  nickel  electrolytically  as  directed  p.  144. 

To  determine  the  iron  in  the  filtrate  from  cobalt  and 
nickel  oxalates,  the  alcohol  and  acetic  acid  are  completely 
removed  by  evaporation,  the  residue  dissolved  in  water,  and 
the  iron  electrolytically  deposited  from  the  solution  of  the 
double  oxalate  (p.  138). 

Determination  of   Potassium,  Sodium,  Lithium,  Calcium,  and 
Magnesium. 

The  mineral  is  dissolved  in  hydrochloric  acid,  evaporated 
to  remove  acid,  and  treated  with  an  excess  of  ammonium 
oxalate.  The  filtrate  from  calcium  oxalate  is  electrolyzed, 
iron,  nickel,  cobalt,  and  copper  separating  as  metals,  man- 
ganese as  dioxide,  and  aluminium  as  hydroxide.  The  filtrate 
from  the  manganese  dioxide  and  aluminium  hydroxide  con- 
tains only  alkalies,  magnesium,  and  a  little  manganese.  It  is 
boiled  to  'remove  the  hydrogen  ammonium  carbonate  formed 
by  the  electrolytic  decomposition  of  ammonium  oxalate,  con- 
centrated to  about  50  cc?  heated  to  boiling,  and  at  least  an 
equal  volume  of  concentrated  acetic  acid  added.  The  pre- 
cipitate consists  of  manganese  and  magnesium  oxalates.  It 
is  filtered  off,  washed  with  a  mixture  of  equal  volumes  of 
alcohol,  acetic  acid,  and  water,  and  ignited.  The  residue  is 
MgO  +  Mn2O3.  It  is  weighed,  dissolved  in  hydrochloric 
acid,  and  the  manganese  determined  by  electrolysis  as 
dioxide  (p.  150). 


APPENDIX.  247 

The  alkalies  are  determined  in  the  filtrate  from  the  man- 
ganese and  magnesium  oxalates.  It  is  evaporated  to  dryness, 
the  ammonium  salts  removed  by  gentle  ignition,  the  residue 
dissolved  in  water,  the  solution  filtered,  and  evaporated  to  t 
dryness  after  addition  of  a  little  hydrochloric  acid.  The 
residue  is  washed  into  a  small  stoppered  flask  with  absolute 
alcohol,  an  equal  volume  of  water-free  ether  added,  and 
allowed  to  stand  twenty-four  hours.  The  solution  is  then 
filtered  from  the  residue,  the  alcohol  and  ether  evaporated, 
and  the  lithium  chloride  converted  into  sulphate  and  weighed. 

The  residue  of  potassium  and  sodium  chlorides  is  dissolved 
in  water,  and  both  metals  directly  determined  as  directed 
p.  229. 

SPHALERITE     (ZINC    BLENDE). 

Constituents :    Zinc    Sulphide,  also    Determinable    Quantities   of 
Iron,  Manganese,  Copper,  Arsenic,  Antimony,  and   Gangue. 

In  most  cases,  it  is  only  necessary  to  determine  the  zinc. 
The  process  is  then  as  follows:  About  0.5  g  of  the  finely 
powdered  mineral  is  digested  with  concentrated  nitric  acid 
till  fully  decomposed,  the  acid  evaporated  off,  and  the  nitrates 
converted  into  chlorides  by  evaporation  with  hydrochloric 
acid.  The  residue  is  dissolved  in  about  25  cc  water  and 
10  cc  hydrochloric  acid,  and  hydrogen  sulphide  passed 
through  the  solution.  The  precipitate  of  sulphides  of  lead, 
copper,  etc.,  is  filtered  off,  washed  with  water  containing 
hydrogen  sulphide  and  hydrochloric  acid,  and  the,  filtrate 
evaporated  -to  dryness.  The  residue  contains  chlorides  of 
zinc,  iron,  manganese,  calcium,  and  magnesium.  It  is  dis- 
solved in  water  with  a  little  hydrochloric  acid,  converted 
into  double  oxalates  (p.  138),  the  calcium  oxalate  filtered  oft', 
and  the  filtrate  electrolyzed.  Zinc  and  iron  separate  at  the 


248        QUANTITATIVE   ANALYSIS   BY   ELECTROLYSIS. 

negative  electrode,  and  manganese,  as  dioxide,  at  the  positive. 
The  two  metals  are  weighed,  dissolved  in  hydrochloric  acid, 
and  the  iron  determined  by  titration  with  potassium  per- 
manganate (p.  191). 

It  is  stated  on  p.  194  that  the  precipitation  of  iron  and 
zinc  from  the  same  solution  is  complete  only  when  there  is 
less  than  one-third  us  much  zinc  as  iron,  arid  that  it  can  be 
successfully  performed,  in  other  cases,  by  adding  a  weighed 
quantity  of  an  iron  salt  before  the  electrolysis. 

Determination  of  Lead,  Copper,  Arsenic,  Antimony,  Zinc,  Iron, 
Manganese,  and  Gangue. 

As  when  zinc  alone  is  to  be  determined,  the  mineral  is 
oxidized  with  nitric  acid,  the  gangue  filtered  off,  and  the  acid 
solution  of  chlorides  treated  with  hydrogen  sulphide.  The 
precipitated  sulphides  are  washed  first  with  hydrogen  sulphide 
water  containing  hydrochloric  acid,  and  afterward  with  pure 
hydrogen  sulphide  water. 

The  antimony  and  arsenic  are  separated  from  lead  and 
copper  by  digestion  with  a  concentrated  solution  of  sodium 
sulphide  ;  the  residue  is  washed  with  the  same  solution,  and 
afterward  with  hydrogen  sulphide  solution.  The  sodium 
sulphide  washings  are  added  to  the  solution  for  determina- 
tion of  arsenic  and  antimony,  and  the  hydrogen  sulphide 
washings  separately  collected. 

The  necessary  amount  of  sodium  hydroxide  is  added  to 
the  sodium  sulphide  solution,  and  the  antimony  and  arsenic 
separated  and  determined  as  directed  p.  224. 

The  sulphides  of  lead  and  copper  are  dissolved  in  nitric 
acid,  and  the  metals  determined  as  directed  p.  213. 

Iron,  zinc,  and  manganese  are  determined  according  to 
previous  directions. 


APPENDIX.  249 

CALAMINE    AND     SMITHSONITE. 

Constituents:  Zinc  (Cadmium),  Copper,  Lead,  Arsenic,  Antimony, 
Iron,  Manganese,  Calcium,  Magnesium,  Silica,  Carbonic  Acid, 
Water. 

Zinc  and  the  other  constituents  are  determined  as  already 
directed.  If  the  mineral  contains  cadmium,  copper  and 
lead  are  first  precipitated  from  the  nitric  acid  solution,  the 
decanted  solution  evaporated  to  dry  ness,  the  cadmium  nitrate 
converted  into  chloride,  and  cadmium  determined  as  directed 
1>.  103. 

ULTRAMARINE. 

Constituents  :    Alumina,  Potassium,   Sodium,  Iron,   Calcium, 
Sulphur,  Silica,   Sulphuric  Acid,   Chlorine. 

A  weighed  portion  of  the  substance  is  dissolved  in  hydro- 
chloric acid,  evaporated  to  dry  ness  to  separate  silica,  the 
residue  dissolved  in  water  with  a  few  drops  of  hydrochloric 
acid,  filtered  from  the  silica,  the  free  acid  neutralized  with 
ammonia,  and  a  great  excess  of  ammonium  oxalate  added. 
The  calcium  oxalate  is  filtered  off,  iron  and  aluminium  deter- 
mined electrolytically,  the  solution  filtered  from  the  alu- 
minium hydroxide,  evaporated  to  dryness,  the  ammonium 
salts  removed  by  gentle  ignition,  the  residue  dissolved  in 
water,  and  the  alkalies  converted  into  chlorides  by  evapora- 
tion with  hydrochloric  acid.  Potassium  and  sodium  are 
determined  as  directed  p.  229. 


250         QUANTITATIVE   ANALYSIS   BY   ELECTROLYSIS. 

REFINERY   SLAG. 

Constituents:  Ferrous  and  Ferric  Oxides,  Metallic  Iron,  Copper, 
Aluminium,  Calcium,  Magnesium,  Silica,  Sulphuric  and  Phosphoric 
Acids, 

A  portion  of  the  substance  (0.5-1  g)  is  dissolved  in 
hydrochloric  acid,  evaporated  to  remove  silica,  the  residue 
dissolved  in  hydrochloric  acid,  evaporated  to  remove  free 
acid,  and  the  metals  converted,  as  usual,  into  double  oxalates. 
Calcium  oxalate  is  filtered  off,  and  the  manganese  in  the 
precipitate  determined  as  directed  p.  239.  The  copper  is 
separated  by  the  action  of  a  weak  galvanic  current  (p.  156), 
and  the  iron,  manganese,  and  aluminium  separated  in  the 
copper-free  solution  as  directed  pp.  194—198. 

For  the  determination  of  magnesium  and  sulphuric  and 
phosphoric  acids,  see  Hematite,  p.  240. 

To  determine  the  metallic  iron,  about  5  g  of  the  finely 
powdered  slag  is  placed  in  a  small  platinum  or  porcelain  dish, 
and  treated  with  an  aqueous  solution  of  copper  sulphate.  A 
quantity  of  metallic  copper  equivalent  to  the  iron  is  precipi- 
tated (CuSO4  +  Fe  =  FeSO4  -f  Cu).  The  decomposition 
is  hastened  by  frequent  stirring ;  the  copper  and  undecom- 
posed  slag  are  finally  filtered  off,  washed  thoroughly,  and 
digested  in  the  water-bath  for  a  long  time  with  nitric  acid. 
In  the  solution,  after  filtration,  the  copper  is  electrolytically 
determined,  and  the  quantity  of  iron  calculated  from  it. 

COPPER     AND    LEAD     SLAGS. 

Constituents  :  Copper,  Lead,  Iron,  Manganese,  Barium,  Calcium, 
Magnesium,  Silica,  Sulphuric  Acid,  Sulphur,  and  ordinarily 
small  quantities  of  Arsenic,  Antimony,  Bismuth,  Cobalt, 
Nickel,  and  Zinc. 

The  slag  is  decomposed  by  digestion  with  nitric  acid, 
evaporated  to  dryness,  the  residue  taken  up  with  water  and 


APPENDIX.  251 

a  little  hydrochloric  acid,  and  the  solution  filtered  from  the 
residue  of  silica  and  barium  sulphate,  which  are  separated  as 
usual.  The  calcium  is  separated  by  adding  ammonium  oxa- 
late  in  great  excess ;  the  calcium  and  the  manganese  it  may 
contain  are  determined  as  directed  p.  239.  Copper  is  then 
precipitated  (p.  155),  and  afterward  iron  and  manganese 
(p.  194:),  and  magnesium  and  sulphuric  acid  are  determined  as 
directed  p.  240. 

In  the  presence  of  arsenic,  antimony,  etc.,  the  hydrochloric 
acid  solution,  after  separation  of  silica,  is  treated,  first  hot 
and  then  cold,  with  hydrogen  sulphide  gas,  and  the  precipi- 
tated sulphides  are  washed  with  hydrogen  sulphide  water, 
and  treated  with  a  concentrated  solution  of  sodium  sulphide. 
The  insoluble  sulphides  of  lead,  copper,  etc.,  are  washed  first 
with  sodium  sulphide,  and  then  with  hydrogen  sulphide 
(see  p.  237),  and  antimony  and  arsenic  are  separated  in  the 
solution  as  directed  on  p.  224. 

The  residue  of  lead  sulphide,  etc.,  is  digested  with  nitric 
acid  till  thoroughly  decomposed,  and  lead  and  copper  sepa- 
rated from  the  solution  as  directed  p.  213.  The  nitric  acid  is 
evaporated  off,  and  bismuth  determined  as  directed  p.  237. 

The  solution  filtered  from  the  hydrogen  sulphide  precipi- 
tate, which  contains  iron,  manganese,  etc.,  is  evaporated 
almost  to  dryness  to  remove  hydrogen  sulphide  and  most  of 
the  hydrochloric  acid,  and  the  metals  finally  converted  into 
double  oxalates.  Calcium  oxalate  is  filtered  oif,  and  the 
precautions  described  on  p.  239  are  observed  in  its  deter- 
mination. By  electrolysis  of  the  filtrate,  iron,  cobalt,  nickel, 
and  zinc  are  obtained  as  metals,  and  manganese,  in  part,  as 
dioxide;  magnesium  remains  in  solution.  The  two  latter 
are  determined  as  directed  p.  241. 

The  iron,  cobalt,  etc. ,  are  dissolved  in  concentrated  hydro- 
chloric acid,  the  solution  evaporated  to  dryness,  the  residue 


252         QUANTITATIVE   ANALYSIS   BY   ELECTROLYSIS. 

chloric  acid,  the  solution  evaporated  to  dryness,  the  residue 
dissolved  in  water  with  a  few  drops  of  acetic  acid,  potassium 
oxalate  added  in  sufficient  quantity  to  form  the  double  oxa- 
lates,  the  solution  diluted  to  25-30  cc,  and  precipitated, 
at  boiling  heat,  with  concentrated  acetic  acid  in  great  excess. 
After  standing  about  six  hours  in  a  warm  place,  the  oxalates 
of  cobalt,  nickel,  and  zinc  are  filtered  off,  washed  with  a 
mixture  of  equal  volumes  of  acetic  acid,  alcohol,  and  water, 
and  the  oxalates  converted,  by  very  gentle  heating,  into 
oxides.  The  mixed  oxides  are  dissolved  in  hydrochloric 
acid,  and  zinc  separated  from  nickel  and  cobalt  as  directed 
on  p.  234.  Iron  is  determined,  in  the  filtrate  from  the  oxa- 
lates, as  directed  on  p.  246. 

BLAST  FURNACE,  CUPOLA,  AND  BESSEMER  SLAGS. 

Constituents  :  Ferrous  and  Ferric  Oxides,  Metallic  Iron,  Man- 
ganese, Aluminium,  Copper,  Lead,  Zinc,  Calcium,  Magnesium, 
Alkalies,  Silica,  Sulphuric  and  Phosphoric  Acids,  Sulphur 
(as  Calcium  Sulphide). 

The  method  of  analysis  is  so  similar  to  the  foregoing  that 
it  needs  only  brief  mention.  The  slag  is  digested  with 
fuming  hydrochloric  acid,  or  aqua  regia,  till  completely 
decomposed,  the  solution  evaporated  on  the  water-bath  to 
dryness,  the  residue  dissolved  in  water  and  a  little  hydro- 
chloric acid,  and  the  silica  filtered  off.  After  conversion  into 
double  oxalates,  the  calcium  oxalate,  which  may  contain 
manganese,  is  filtered  off  (p.  239),  copper  and  lead  first 
precipitated  (p.  213),  then  iron  and  zinc  with  aluminium  and 
the  rest  of  the  manganese ;  iron  and  zinc  are  determined  as 
directed  p.  193,  and  manganese,  aluminium,  and  magnesium 
as  directed  p.  241.  The  alkalies  and  sulphuric  and  phos- 
phoric acids  are  determined  as  heretofore  directed. 


APPENDIX.  253 

ZIRCON. 

Constituents :    Zirconia,  Iron   Oxide,  Lime,   Silica. 

The  mineral  is  decomposed  by  long-continued  fusion 
with  sodium  carbonate,  the  fused  mass  dissolved  in  hydro- 
chloric acid,  the  solution  evaporated  to  dryness,  the  residue 
taken  up  with  water  acidified  with  hydrochloric  acid,  the 
silica  filtered  off,  and  the  filtrate  treated  with  a  great  excess 
of  ammonium  oxalate.  To  overcome  the  injurious  effect  of 
sodium  chloride,  about  10  g  ammonium  oxalate  must  be 
dissolved  by  heating  in  the  solution  diluted  to  about  200  cc. 
The  separation  of  iron  and  zirconium  is  carried  out  under  con- 
ditions similar  to  those  given  for  Iron-Beryllium,  p.  201.  If 
calcium  is  present,  the  calcium  oxalate  precipitate  is,  of 
course,  to  be  filtered  off  before  electrolysis,  and  determined. 

ARSENOPYRITE. 
Iron,  Arsenic,  Antimony,  Sulphur,  Gangue. 

A  portion  of  the  finely  powdered  mineral  is  oxidized  with 
aqua  regia  till  fully  decomposed,  the  gangue  filtered  off,  and 
the  solution  evaporated  to  dryness.  The  chlorides  are  con- 
verted into  sulphates  by  moistening  and  heating  with  sul- 
phuric acid,  water  is  added,  the  solution  heated  to  70°-80°, 
and  hydrogen  sulphide  passed  till  it  has  cooled  completely. 
After  -standing  some  twelve  hours  at  a  moderate  heat,  the 
sulphides  of  arsenic  and  antimony  are  filtered  off,  and  sepa- 
rated as  directed  p.  224. 

To  determine  the  iron,  the  hydrogen  sulphide  is  driven  off 
from  the  solution,  which  is  then  treated  as  directed  p.  138. 


254        QUANTITATIVE  ANALYSIS   BY   ELECTROLYSIS. 

CHALCOPTRITE   (COPPER   PYRITES). 

Constituents:    Copper,   Iron,   Sulphur,   Gangue. 

The  mineral  is  oxidized  with  nitric  acid,  the  gangue 
filtered  off,  and  copper  precipitated  in  the  filtrate  (p.  156). 

To  determine  iron,  nitric  acid  is  removed  by  evaporation, 
concentrated  hydrochloric  acid  added,  the  solution  again 
evaporated,  and  finally  iron  is  precipitated,  after  formation 
of  the  double  oxalate,  according  to  directions  on  p.  138. 

Sulphur*  may  be  determined  in  the  same  portion  by  pre- 
cipitating sulphuric  acid  with  barium  chloride,  and  removing 
the  excess  of  the  latter  by  careful  addition  of  sulphuric  acid. 
Copper  is  then  separated  from  iron  in  sulphuric  acid  solution, 
and  the  latter  determined  as  usual. 

As  already  stated  on  p.  157,  copper  cannot  be  precipitated 
from  either  nitric  or  sulphuric  acid  solution  in  the  presence 
of  any  considerable  quantity  of  arsenic  and  antimony  without 
being  contaminated  by  them. 

If  only  the  copper  is  to  be  determined,  the  nitric  acid 
solution  of  the  mineral  is  evaporated  to  dry  ness,  the  residue 
dissolved  in  water  with  a  little  acetic  acid,  and  potassium 
oxalate  added  in  excess.  The  solution  is  filtered  hot  from 
the  gangue,  the  residue  washed  with  water  containing  potas- 
sium oxalate,  and  the  filtrate  brought  to  a  volume  of  about 
50  cc.  After  cooling,  almost  all  the  copper  crystallizes  out 
as  potassium  copper  oxalate ;  the  rest  is  precipitated  by 
addition  of  much  concentrated  acetic  acid.  The  precipitate 
is  washed  with  a  mixture  of  equal  volumes  of  water,  acetic 
acid,  and  alcohol,  dissolved  in  ammonium  oxalate,  and  elec- 
trolyzed. 

If  arsenic  and  antimony  are  present  in  larger  proportion, 
the  finely  pulverized  mineral  is  mixed  with  four  times  its 


APPENDIX.  255 

weight  of  ammonium  chloride,  and  heated  gently  in  a 
covered  crucible.  Arsenic  and  antimony,  and  the  greater 
part  of  the  iron  are  volatilized  as  chlorides.* 

The  residue  is  dissolved  in  nitric  acid,  and  treated  as 
before. 

NICKEL    MATTE.      COPPER    MATTE. 

Nickel,  Cobalt,  Zinc,  Iron,  Copper,  Lead,  Arsenic,  Antimony, 
Sulphur,  Gangue. 

The  substance  is  decomposed  with  aqua  regia,  evaporated 
to  dryness,  the  residue  dissolved  in  hydrochloric  acid,  and 
filtered  from  the  gangue.  In  this  solution,  the  metals  pre- 
cipitable  by  hydrogen  sulphide  are  precipitated  by  heating  to 
70°-80°,  and  passing  hydrogen  sulphide  gas  till  the  solution 
becomes  cold.  The  precipitate  is  filtered  off,  washed  first 
with  a  solution  containing  hydrogen  sulphide  and  hydro- 
chloric acid,  then  with  pure  hydrogen  sulphide  solution,  and 
treated  with  a  concentrated  solution  of  sodium  sulphide  as 
directed  p.  222,  and  the  arsenic  and  antimony  separated  and 
determined  as  directed  p.  224. 

The  sulphides  of  lead  and  copper  left  undissolved  by 
sodium  sulphide  are  digested  with  nitric  acid,  and  deter- 
mined as  directed  p.  213.  The  filtrate  from  the  hydrogen 
sulphide  precipitate  is  evaporated  to  dryness  to  remove 
hydrogen  sulphide  and  hydrochloric  acid,  the  residue  dis- 
solved in  water  with  a  little  acetic  acid,  potassium  oxalate 
added  in  excess,  and  the  solution  of  50-100  cc  precipitated 
boiling  hot  with  a  great  excess  of  concentrated  acetic  acid 
(at  least  an  equal  volume).  The  precipitate  of  nickel,  cobalt, 
and  zinc  oxalates  is  filtered  off,  washed  with  a  mixture  of 
equal  volumes  of  alcohol,  acetic  acid,  and  water  (p.  214), 

*  Classen,  Zts.  anal.  Ch.,  18,  388. 


256         QUANTITATIVE  ANALYSIS   BY    ELECTROLYSIS. 

dried,  and  converted,  by  gentle  ignition,  into  the  oxides.  The 
residue  is  dissolved  in  hydrochloric  acid,  and  zinc,  cobalt, 
and  nickel  separated,  and  determined  as  directed  pp.  204  and 
245. 

Iron  is  determined,  in  the  filtrate  from  the  mixed  oxalates, 
as  directed  p.  245. 

COPPER  SPEISS,  LEAD  SPEISS. 

Antimony  or  Arsenic  Compounds  of  Iron,  Cobalt,  and  Nickel, 
together  with  Sulphur  Compounds  of  Copper,  Lead,  Silver, 
Bismuth,  Iron,  and  Zinc. 

It  is  best  to  decompose  the  finely  powdered  substance  in 
a  suitable  apparatus  *  with  chlorine  gas,  volatilizing  arsenic, 
antimony,  iron,  and  zinc,  as  chlorides,  and  collecting  them  in 
a  receiver  containing  equal  volumes  of  hydrochloric  and 
tartaric  acids.  The  free  chlorine  is  expelled,  by  heat,  from 
the  solution  in  the  receiver,  and  hydrogen  sulphide  passed 
into  the  still  hot  solution  until  it  cools.  The  sulphides  are 
filtered,  washed,  treated  with  sodium  sulphide,  and  arsenic 
and  antimony  determined  in  the  solution,  as  directed  p.  224. 
The  insoluble  sulphides  of  iron  and  zinc  are  dissolved  in 
hydrochloric  acid,  evaporated  to  dryness,  the  residue  dis- 
solved in  water  with  a  few  drops  of  hydrochloric-  acid,  and 
iron  and  zinc  determined  as  directed  p.  193. 

After  the  decomposition  with  chlorine,  the  non-volatile 
chlorides  of  copper,  lead,  silver,  bismuth,  cobalt,  and  nickel, 
and  a  part  of  the  iron  and  zinc,  remain  in  the  bulb.  They 
are  dissolved  in  dilute  hydrochloric  acid,  and  lead,  copper, 
silver,  and  bismuth  precipitated  with  hydrogen  sulphide. 
The  sulphides  are  digested  with  nitric  acid  till  completely 

*  Classen,  Quantitative  Analyse,  4th  ed.   p.  187. 


APPENDIX.  257 

dissolved,  and  copper  and  silver  precipitated  as  metals,  and 
lead  as  peroxide,  by  electrolysis.  Copper  and  silver  are 
separated  as  directed  p.  216,  and  bismuth  from  some  residual 
lead  as  directed  p.  237. 

The  separation  of  cobalt  and  nickel  from  iron  and  zinc  is 
given  on  pp.  204  and  244. 

PYRARGYRITE. 
Silver,  Antimony   (Arsenic),   Sulphur,  Gangue. 

The  mineral  may  be  decomposed  by  chlorine  gas,  or  by 
heating  with  anhydrous  sodium  thiosulphate.  In  the  former 
case,  the  chlorides  of  antimony  and  arsenic  (and  sulphur)  go 
into  solution,  while  silver  chloride  remains  in  the  bulb  tube. 
In  the  latter  case,  when  the  fused  mass  is  treated  with  water, 
silver  sulphide  remains  unclissolved,  and  may  be  dissolved  in 
nitric  acid,  and  the  silver  deposited,  as  metal,  from  the  solu- 
tion (p.  174). 

To  determine  antimony,  and  separate  it  from  arsenic,  the 
solution  of  sodium  pentasulphide  is  oxidized  with  hydrogen 
peroxide,  evaporated,  and  treated  as  in  the  determination  of 
antimony  in  presence  of  tin  (p.  225). 

TETRAHEDRITE. 

Copper,  Antimony,  Arsenic,  Silver,  Lead,  Iron,  Zinc,  Sulphur, 

Gangue. 

The  mineral  may  be  decomposed  as  heretofore  described. 
When  chlorine  gas  is  used,  the  receiver  contains  chlorides  of 
antimony,  arsenic,  iron,  and  zinc  (and  sulphur)  ;  the  bulb- 
tube,  copper,  lead,  silver,  and  gangue,  with  a  portion  of  the 
iron  and  zinc.  The  metals  are  separated  as  already  described* 


258         QUANTITATIVE   ANALYSIS   BY   ELECTROLYSIS. 

FURNACE    "SOWS." 

Alloys  of  Iron  (the  principal  constituent),  Copper,  Silver,  Lead, 
Molybdenum,  Vanadium,  Cobalt,  Nickel,  and  Zinc,  with 
Sulphides  and  Phosphides  of  these  Metals,  and  varying 
amounts  of  Carbonic  Acid  and  Silica. 

The  substance  is  best  decomposed  by  chlorine  gas.  The 
quantity  of  iron  is  so  great,  however,  that  two  bulb-tubes 
of  the  most  infusible  glass  should  be  used,  in  the  second  of 
which  is  deposited  most  of  the  iron  chloride.  The  substance 
is  heated  in  a  stream  of  chlorine  as  long  as  iron  chloride 
sublimes  ;  then  it  is  certain  that  all  the  molybdenum 
chloride  will  have  been  carried  over  into  the  receiver,  which 
also  contains  vanadium,  sulphur,  and  phosphorus  chlorides. 
Hydrogen  sulphide  is  passed  into  the  solution  collected  in 
the  receiver  until  the  supernatant  liquid  is  colorless.  The 
precipitate  of  molybdenum  sulphide  is  filtered  off,  washed, 
oxidized  with  nitric  acid,  the  solution  supersaturated  with 
ammonia,  and  molybdenum  oxide  precipitated  by  electrolysis. 

The  filtrate  from  molybdenum  sulphide  contains  vana- 
dium and  iron.  Hydrogen  sulphide  and  hydrochloric  acid 
are  evaporated  off,  double  oxalates  formed,  and  the  two 
metals  separated  electrolytically,  according  to  the  method 
given  for  the  separation  of  Beryllium-Iron,  p.  201.  To 
determine  vanadium  in  the  solution  decanted  from  the  iron, 
it  is  evaporated  to  dryness,  the  ammonium  salts  driven  off 
by  careful  ignition,  and  the  residue  of  vanadium  oxide 
converted,  by  fusion  with  potassium  nitrate,  into  potassium 
vanadate.  The  fused  mass  is  dissolved  in  water,  nitric  acid 
added  not  to  acid  reaction,  then  a  concentrated  solution  of 
ammonium  chloride,  and  then  alcohol  in  the  proportion  of 
one  volume  to  three  of  the  solution.  After  standing  forty- 
eight  hours,  the  ammonium  vanadate  is  filtered  off,  and 
washed  with  a  concentrated  solution  of  ammonium  chloride. 


APPENDIX.  259 

and  then  with  alcohol.  The  salt  is  heated  first  in  the  air, 
then  in  a  stream  of  oxygen,  and  leaves  a  residue  of  pure 
vanadic  acid  which  is  weighed. 

The  chlorides  remaining  in  the  bulb-tube  are  heated  with 
hydrochloric  acid  ;  a  residue  of  silver  chloride  and  carbon 
remains.  It  is  heated  with  potassium  cyanide,  the  carbon 
filtered  off,  and  the  silver  determined  by  electrolysis. 

The  methods  of  separation  and  determination  of  the 
metals  in  the  hydrochloric  acid  solution  have  already  been 
given. 

STIBNITE    (ANTIMONY    GLANCE). 

Constituents:    Antimony  and  Sulphur,  and  usually  small 
quantities  of  Iron,  Lead,  Copper,  and  Arsenic. 

The  simplest  method  of  analyzing  the  mineral  is  to  mix 
with  four  or  five  times  its  weight  of  anhydrous  sodium 
thiosulphate,  and  heat  for  a  long  time  in  a  covered  crucible 
(p.  237).  The  fused  mass  is  extracted  with  water;  the 
solution  contains  antimony  and  arsenic,  and  is  treated  for 
decomposition  of  sodium  pentasulphide  and  determination  of 
the  two  metals  as  directed  p.  224 ;  the  undissolved  sulphides 
of  lead,  copper,  and  iron  are  oxidized  with  nitric  acid,  and 
the  metals  separated  according  to  foregoing  directions. 

ULLMANITE. 
Antimony,  Nickel,  and  Sulphur. 

The  finely  powdered  mineral  is  decomposed  in  a  stream 
of  chlorine  (p.  256),  all  the  antimony  passing  into  the 
receiver  as  chloride,  and  nickel  chloride  remaining  in  the 
bulb-tube.  The  latter  is  determined  by  dissolving  the  con- 
tents of  the  bulb  in  hydrochloric  acid,  evaporating,  convert- 
ing into  the  double  oxalate,  and  precipitating  by  electrolysis. 


260         QUANTITATIVE   ANALYSIS   BY    ELECTROLYSIS. 

Antimony  is  precipitated,  as  sulphide,  by  passing  hydro- 
gen sulphide  gas  into  the  solution,  in  hydrochloric  and  tar- 
taric  acids,  dissolved  in  concentrated  sodium  sulphide,  the  solu- 
tion diluted  with  water  and  submitted  to  electrolysis  (p.  179). 
If  the  mineral  contains  iron,  it  passes  over,  as  chloride,  into 
the  receiver ;  it  may  be  determined,  in  the  filtrate  from 
antimony  sulphide,  after  supersatu ration  with  ammonia,  by 
precipitation  as  sulphide  with  ammonium  sulphide.  The 
sulphide  thus  obtained  is  dissolved,  converted  into  the 
double  oxalate,  and  iron  determined  electrolytically  (p.  138). 

The  analysis  is  made  more  simply  if  the  mineral  is 
decomposed  by  heating  with  sodium  thiosulphate ;  when  the 
proportion  of  antimony  is  large,  it  is  necessary  to  repeat 
the  process  with  the  residual  nickel  sulphide.  Antimony  is 
determined  in  the  aqueous  solution  of  the  fused  mass  as 
directed  p.  181.  If,  on  treatment  with  hydrogen  peroxide, 
or  addition  of  sodium  monosulphide,  some  nickel  sulphide 
separates,  it  is  added  to  the  principal  portion. 

The  sulphides  of  iron  and  nickel  are  oxidized  with  nitric 
acid,  the  nitrates  converted  into  chlorides,  and  the  two 
metals  separated  as  directed  (p.  192). 

BOURNONITE. 

Antimony,  Lead,  Copper  (Iron),  and  Sulphur. 

The  finely  powdered  mineral  is  heated  either  with 
chlorine  or  anhydrous  sodium  thiosulphate,  and  the  analysis 
conducted  as  already  described. 

ZINKENITE. 
Antimony,  Lead  (Silver,  Copper,  Iron),  Sulphur. 

The  mineral  is  most  simply  decomposed  by  heating 
with  anhydrous  sodium  thiosulphate.  After  extracting  with 


APPENDIX.  261 

water,  the  residue  of  undissolved  sulphides  is  dried,  the 
filter  burnt,  and  fusion  with  thiosulphate  repeated.  Anti- 
mony is  determined  according  to  directions  on  p.  179.  The 
sulphides  of  lead,  silver,  etc.,  are  oxidized  with  nitric  acid; 
copper  and  silver  precipitated  electrolytically,  and  separated 
as  directed  p.  216.  A  portion  of  the  lead  is  separated,  as 
peroxide,  by  the  electrolysis  of  the  nitric  acid  solution,  and 
is  determined  as  such.  The  rest  is  precipitated  with  hydro- 
gen sulphide,  the  filtrate  neutralized  with  ammonia,  ammo- 
nium oxalate  added,  and  iron  determined  by  electrolysis. 

LINN-3EIITB. 
Constituents :    Cobalt  and  Sulphur. 

The  analysis  of  this  mineral  is  very  simple.  It  is  dis- 
solved in  aqua  regia,  the  free  acid  evaporated  off,  and 
chlorides  formed  by  repeated  evaporation  with  hydrochloric 
acid. 

The  aqueous  solution  of  the  residue  is  treated  with  an 
excess  of  ammonium  oxalate,  and  cobalt  precipitated  electro- 
lytically (p.  141).  If  iron  is  present,  the  two  metals  are 
separated  as  directed  p.  191. 

In  the  solution  decanted  from  the  metallic  cobalt,  ammo- 
nium carbonate  is  decomposed  by  boiling,  hydrochloric  acid 
is  added,  and  the  sulphur  determined  by  precipitation  with 
barium  chloride. 

COBALTITE. 
Cobalt,  Iron  (Copper,  Antimony),  Arsenic,  and  Sulphur. 

The  mineral  may  be  decomposed  by  heating  with  nitric 
acid,  or  with  sodium  thiosulphate.  If  nitric  acid  is  used, 
the  free  acid  is  evaporated  off,  and  the  nitrates  converted 


262         QUANTITATIVE  ANALYSIS   BY   ELECTROLYSIS. 

into  chlorides.  In  the  hydrochloric  acid  solution,  arsenic, 
antimony,  and  copper  are  precipitated,  as  sulphides,  by  pass- 
ing hydrogen  sulphide  into  the  hot  solution  till  it  cools;  the 
sulphides  are  digested  with  sodium  sulphide,  and  the  solution 
treated  as  directed  p.  224.  The  residue  of  copper  sulphide  is 
dissolved  in  nitric  acid,  and  the  copper  separated  by  elec- 
trolysis (p.  156).  The  filtrate  from  the  hydrogen  sulphide 
precipitate  is  freed  from  hydrogen  sulphide  and  hydro- 
chloric acid,  and  iron  and  cobalt  are  separated  as  directed 
p.  191. 

If  the  mineral  is  heated  with  anhydrous  sodium  thio- 
sulphate,  and  extracted  with  water,  antimony  and  arsenic  go 
into  solution,  and  are  determined  as  directed  p.  224. 

The  sulphides  insoluble  in  water  are  dissolved  in  nitric 
acid,  and  copper  first  precipitated  (p.  156);  the  nitrates  are 
then  converted  into  chlorides,  and  cobalt  and  iron  deter- 
mined (p.  191). 

Finally,  arsenic  and  antimony  may  also  be  determined  by 
removing  the  arsenic  first.  The  nitric  acid  solution  is  heated 
with  sulphuric  acid  to  convert  nitrates  into  sulphates.  The 
arsenic  is  driven  off  from  this,  as  chloride,  by  treatment  with 
ferrous  chloride  or  sulphate,  and  distillation  in  a  stream  of 
hydrochloric  acid  (p.  226).  To  determine  antimony,  the 
residue  in  the  flask  is  saturated  with  hydrogen  sulphide,  and 
filtered ;  the  precipitate  is  washed,  and  treated  with  sodium 
sulphide  (p.  227). 


COBALTIFEROUS     ARSENOPYRITE. 
Cobalt,  Iron,  Arsenic,  and  Sulphur. 

The   mineral   is   analyzed   in   the   same   manner   as   co- 
baltite. 


APPENDIX.  263 

CERUSSITE. 

« 

Lead,  Iron,  Calcium,   Carbonic  Acid. 

The  pulverized  mineral  is  dissolved  by  heating  with  nitric 
acid,  and  the  lead  determined,  as  peroxide,  by  connecting 
the  platinum  dish  with  the  positive  pole  of  the  battery 
(p.  169V 

The  solution  decanted  from  the  lead  peroxide  is  evapo- 
rated to  dryness  with  hydrochloric  acid,  the  residue  taken  up 
with  water  and  a  few  drops  of  hydrochloric  acid,  treated 
with  ammonium  oxalate  in  great  excess,  calcium  oxalate 
filtered  off,  and  iron  determined  electrolytically  in  the  filtrate 
(p.  138). 

GALENA. 

Lead  (Antimony,  Arsenic,  Copper,  Silver,  Gold,  Zinc,  Iron), 
Sulphur,  Gangue. 

Galena  rich  in  antimony  is  decomposed  either  by  chlorine, 
or  by  heating  with  anhydrous  sodium  thiosulphate.  When 
decomposed  with  chlorine,  the  receiver  contains  antimony, 
arsenic,  iron,  and  zinc.  These  metals  are  separated  as 
directed  p.  256.  The  chlorides  remaining  in  the  bulb-tube 
are  dissolved  in  hot  dilute  hydrochloric  acid,  and  evaporated 
on  the  water-bath,  with  addition  of  sulphuric  acid,  till  the 
hydrochloric  acid  is  all  driven  off.  The  residue  is  diluted 
with  water,  one-third  its  volume  of  alcohol  added  to  the 
solution,  and  the  lead  sulphate  filtered  off.  In  the  filtrate, 
copper  and  silver  are  precipitated  with  hydrogen  sulphide, 
the  sulphides  oxidized  with  nitric  acid,  and  determined  as 
directed  p.  216.*  The  filtrate  from  the  hydrogen  sulphide 

*  AS  silver  and  gold  are  present  only  in  small  quantities,  they  are 
ordinarily  determined  by  cupellation. 


264         QUANTITATIVE   ANALYSIS   BY    ELECTKOLYSIS. 

precipitate  is  evaporated,   and  iron  and  zinc  determined  as 
directed  p.  193. 

By  heating  galena  with  sodium  thiosulphate,  and  extract- 
ing with  water,  antimony  and  arsenic  (and  gold)  are  found 
in  the  solution,  and  are  separated  as  already  directed ;  the 
sulphides  of  lead,  silver,  copper,  zinc,  and  iron  remain  undis- 
solved.  The  proportion  of  lead  is  so  great  that  it  cannot 
well  be  determined,  as  dioxide,  in -nitric  acid  solution  ;  it  is 
converted  into  sulphate,  and  the  analysis  completed  as  before. 

PYROMORPHITE. 

Lead  Phosphate  and  Chloride,  sometimes  Sulphate  and 
Arsenate. 

The  finely  pulverized  mineral  is  digested  with  nitric  acid, 
and  evaporated  to  dryness  with  hydrochloric  acid.  The 
residue  is  moistened  with  hydrochloric  acid,  dissolved  in  hot 
water,  the  clear  nitrate  poured  off,  and  the  lead  chloride, 
which  had  crystallized  out,  brought  into  solution  by  repeated 
boiling  with  water.  Lead  and  arsenic  are  precipitated  by 
passing  hydrogen  sulphide  into  the  hot  solution  till  it  cools, 
filtered  hot  after  long  standing,  and  the  precipitate  washed 
and  digested  with  sodium  sulphide.  Arsenic  is  determined 
in  the  solution  as  directed  p.  223.  The  lead  sulphide  is 
oxidized  with  nitric  acid,  and  lead  determined,  as  peroxide, 
as  directed  p.  168.  Phosphoric  acid  is  determined,  in  the 
usual  way,  in  the  filtrate  from  the  hydrogen  sulphide 
precipitate. 

LEAD    MATTE. 
Lead,  Copper,  Iron  (Silver,  Antimony,  Nickel,  Zinc),  Sulphur. 

If  the  mineral  is  decomposed  by  heating  in  chlorine,  iron 
and  antimony  pass  over  into  the  receiver.  The  analysis  is 
conducted  according  to  directions  for  copper  or  lead  speiss. 


APPENDIX.  265 

CINNABAR. 

Constituents :    Mercury,  Manganese,   Copper,  Alumina,  Iron, 
Calcium,  Sulphur. 

The  mineral  is  decomposed  by  heating  with  aqua  regia, 
the  solution  evaporated  on  the  water-bath,  and  the  metals 
converted  into  nitrates  by  repeated  evaporation  with  nitric 
acid.  Mercury  and  copper  are  precipitated  from  the  nitric 
acid  solution  (p.  175),  the  two  metals  redissolved  in  nitric 
acid,  converted  into  the  double  cyanides,  and  determined 
according  to  the  directions  on  p.  216.  The  small  amount  of 
manganese  present  is  precipitated,  as  dioxide,  in  the  elec- 
trolytic process,  and  may  be  weighed  as  such. 

To  determine  iron,  aluminium,  and  calcium,  the  solution 
decanted  from  the  metals  is  evaporated  to  dryness  on  the 
water-bath,  the  nitric  acid  removed  by  repeated  evaporation 
with  hydrochloric  acid,  the  weak  acid  solution  of  the  residue 
treated  with  ammonium  oxalate  in  great  excess,  calcium 
oxalate  filtered  off,  and  iron  and  aluminium  determined  as 
directed  p.  197. 

SOFT  LEAD  (CRUDE   LEAD). 

In  addition  to  Lead,  small  quantities  of  Silver,  Copper,  Bismuth,  Anti- 
mony, Arsenic,  Cadmium,  Iron,  Zinc,  Cobalt,  Nickel. 

According  to  the  purity  of  the  metal,  200  to  500  grams 
are  taken.  The  weighed  quantity,  cleaned  and  rolled  into 
thin  plates,  is  digested  with  a  mixture  of  about  250  cc  con- 
centrated nitric  acid,  sp.  gr.  1.4,  and  500-600  cc  water.  The 
solution  is  hastened  by  careful  heating  on  a  sand  or  water 
bath.  If  the  acid  works  very  actively,  the  flask  is  removed 
from  the  bath,  but  not  long  enough  for  crystals  of  lead 


266         QUANTITATIVE   ANALYSIS    BY    ELECTROLYSIS. 

nitrate  to  separate  from  the  cooled  solution.  If  there  is  not 
more  than  0.02-0.03  per  cent  of  antimony,  a  perfectly  clear 
solution  is  finally  obtained.  If  the  filtrate  is  turbid  from  the 
presence  of  lead  antimonate,  the  precipitate  is  filtered  off, 
and  washed  thoroughly  with  water  (residue  I). 

The  nitric  acid  solution  is  transferred  into  a  2 -liter 
measuring-flask,  about  170  cc  of  dilute  sulphuric  acid  are 
added  (1  part  concentrated  sulphuric  acid  and  2  parts 
water),  thus  precipitating  all  the  lead  as  sulphate,  and  the 
flask  is  filled  to  the  mark.  The  contents  of  the  flask  are 
thoroughly  shaken,  the  precipitate  allowed  to  settle,  and  the 
greater  part  of  the  solution  siphoned  off,  taking  care  not  to 
disturb  the  lead  sulphate. 

1,750  cc  of  the  clear  solution  are  evaporated  till  white 
fumes  of  sulphuric  acid  appear;  50-60  cc  of  water  are 
added  after  cooling,  and  the  small  amount  of  lead  sulphate 
that  may  remain  undissolved  is  filtered  off.  As  this  latter 
may  contain  antimony,  it  is  digested  with  concentrated 
sodium  sulphide,  and  the  solution  siphoned  off  (solution  I). 

The  filtrate  from  lead  sulphate  is  heated  to  about  70°, 
and  hydrogen  sulphide  passed  in  till  it  cools.  When  the 
precipitate,  after  long  standing  on  the  sand-bath,  has  com- 
pletely subsided,  it  is  filtered  off,  washed  thoroughly  with 
water  containing  hydrogen  sulphide,  and  digested  with  a 
concentrated  solution  of  sodium  sulphide.  The  residue 
marked  I  is  also  treated  with  sodium  sulphide,  and  the 
dissolved  portion,  together  with  solution  I,  added  to  the 
principal  solution.  Antimony  and- arsenic  are  then  separated 
and  determined  as  directed  p.  224. 

The  sulphides  insoluble  in  sodium  sulphide  (copper, 
cadmium,  etc.)  are  digested  with  nitric  acid  till  completely 
oxidized,  and  copper  and  silver  are  separated  from  the 
solution,  as  metals,  by  electrolysis,  and  any  remaining  lead 


APPENDIX.  267 

as   peroxide.      The    copper   and    silver   are   separated    and 
determined  as  directed  p.  216. 

To  determine  bismuth  arid  cadmium,  the  nitric  acid  is 
completely  removed  by  evaporation,  the  residue  dissolved  in 
water  with  a  few  drops  of  dilute  hydrochloric  acid,  potassium 
cyanide  added,  and  the  solution  gently  heated  on  the  water- 
bath  ;  the  potassium  bismuth  cyanide  is  filtered  off  and  washed 
with  water.  The  bismuth  may  then  be  determined  gravi- 
metrically. 

Cadmium  can  be  directly  electrolyzed  from  the  solution 
of  cadmium  potassium  cyanide  (p.  165). 

The  filtrate  from  the  original  hydrogen  sulphide  precipi- 
tate, which  contains  zinc,  iron,  cobalt,  nickel,  etc.,  is  heated 
to  boiling  and  oxidized  with  bromine- water.  An  excess  of 
sodium  hydroxide  is  added,  and  the  metals,  with  the  excep- 
tion of  zinc,  are  precipitated  as  hydroxides.  The  solution  is 
filtered  off,  and  the  zinc  precipitated  by  electrolysis,  either 
directly  from  the  filtrate,  or  after  being  first  converted  into 
some  other  salt.  The  hydroxides  are  dissolved  in  dilute  sul- 
phuric acid,  the  iron  is  precipitated  with  ammonium  hydroxide 
and  determined  either  gravimetrically  or  by  electrolysis.  The 
nickel  and  cobalt  are  determined  in  the  solution,  from  which 
the  iron  has  been  removed,  by  electrolysis  under  the  condi- 
tions given  on  p.  144. 

In  calculating  the  analysis,  the  space  occupied  by  the  lead 
sulphate  in  the  solution  is  to  be  taken  into  account.  100  g 
lead  converted  into  sulphate  occupy  a  space  of  23  cc;  200  g, 
therefore,  46  cc.  Accordingly  in  making  the  calculation, 
1750  cc  are  to  be  reduced,  not  to  2000  cc,  but  to  2000  —  46 
=  1954  cc,  or  to  179.12  g  lead. 

Crude  lead  is  also  analyzed  by  the  foregoing  method; 
10  to  50  g  is  a  sufficient  quantity  for  the  analysis. 


268         QUANTITATIVE   ANALYSIS   BY    ELECTKOLYSIS. 


ANTIMONY. 

Metallic  antimony  may  be  treated  in  the  same  way  as 
hard  lead,  p.  237. 

SPELTER   (CRUDE   ZINC). 

Zinc   and   determinable   quantities   of  Lead,   Iron,    Cadmium,  Arsenic, 
Antimony,  Tin,  and  Copper. 

In  the  analysis  of  crude  metals,  the  determination  of  the 
impurities  is  of  more  importance  than  that  of  the  metal.  As 
the  quantity  of  other  metals  is  so  small,  it  is  necessary  to 
dissolve  a  large  quantity  of  zinc.  According  to  its  purity, 
25  to  100  g  are  taken,  and  dissolved,  in  a  flask,  by  gradual 
addition  of  hydrochloric  acid,  some  zinc,  however,  being  left 
undissolved.  If  the  zinc  comes  in  sticks,  a  stick  may  be 
fastened  to  a  platinum  wire,  and  dipped  partly  into  the 
solution,  and  the  undissolved  zinc  removed,  cleaned,  and 
weighed. 

In  both  cases,  zinc  only  goes  into  solution ,  the  other 
metals,  with  the  exception  of  arsenic  and  antimony,  being 
left  as  spongy  masses.  It  is  necessary,  however,  to  filter  the 
solution  of  zinc  at  once,  and  to  wash  the  residue.  The 
latter  is  digested  with  nitric  acid,  and  carbon  and  silica,  with 
all  the  tin  oxide  and  small  quantities  of  antimony  (most  of 
it  was  volatilized  during  the  solution  in  hydrochloric  acid) 
and  lead,  remain  undissolved.  To  determine  the  tin,  the 
residue  is  heated  with  concentrated  hydrochloric  acid,  carbon 
and  silica  are  filtered  off,  the  filtrate  is  evaporated  to  dry  ness, 
and  the  residue  digested  with  a  concentrated  solution  of 
sodium  sulphide.  The  antimony  and  tin  in  the  filtered 
solution  are  separated  as  directed  p.  221. 

The  nitric  acid  solution  of  the  metals  is  evaporated,  the 


APPENDIX.  269 

residue  dissolved  in  dilute  hydrochloric  acid,  diluted  with 
water,  and  hydrogen  sulphide  passed  into  the  hot  solution 
till  it  lias  thoroughly  cooled.  The  precipitate,  after  settling, 
is  filtered  off,  washed  with  water,  and  digested  with  concen- 
trated sodium  sulphide.  The  sulphides  of  lead,  copper,  etc., 
remain,  are  dissolved  in  nitric  acid,  and  separated  as  in  the 
analysis  of  soft  lead. 

The  filtrate  from  the  hydrogen  sulphide  precipitate  is 
heated  to  boiling  and  treated  with  bromine- water ;  and  the 
metals  present,  iron,  zinc,  nickel,  cobalt,  etc.,  determined  as 
in  the  analysis  of  soft  lead. 

Antimony  and  arsenic  must  be  determined  in  a  separate 
portion  of  zinc,  which  is  dissolved  in  aqua  regia.  The  aqua 
regia  is  evaporated  off,  the  residue  treated  with  concentrated 
hydrochloric  acid,  and  again  evaporated,  and  finally  dissolved 
in  dilute  hydrochloric  acid.  Hydrogen  sulphide  is  passed 
into  the  solution  as  before,  the  sulphides  are  filtered  off  after 
long  standing,  washed  thoroughly  with  water,  and  digested 
with  a  concentrated  solution  of  sodium  sulphide.  Antimony 
and  arsenic  and  tin,  if  present,  are  determined  as  directed 
pp.  225  ff. 


BLISTER    COPPER* 

Copper,  Iron,  Lead,   Silver,  Antimony,  Arsenic,  Bismuth,  Zinc, 
Nickel,  Cobalt 


grams  of  blister  copper  must  be  taken  to  determine 
the  impurities  ;   it  is  analyzed  in  two  separate  portions  of 

*  Process  of  analysis  partly  after  W.  Hampe  (  "  Beitriige  zur  Metal  lurgie 
des  Kupfers"),  Zts.  fiir  Berg-,  Hiltten-  und  Salinenwesen,  27,  205. 


270         QUANTITATIVE   ANALYSIS    BY   ELECTROLYSIS. 

25  g  each.  Each  portion  of  25  g  of  bright  copper  cut- 
tings is  digested  with  a  mixture  of  about  175  cc  nitric 
acid,  sp.  gr.  1.2,  and  200  cc  water,  till  no  metallic  residue 
is  left ;  and  after  cooling,  whether  the  solution  is  clear  or 
not,  25  cc  of  concentrated  sulphuric  acid  are  carefully  added. 
The  solution  is  evaporated  on  the  water-bath,  and  heated  on 
the  sand-bath  till  the  excess  of  sulphuric  acid  is  driven  off. 
After  cooling,  20  cc  nitric  acid  is  added,  the  solution  is 
diluted  with  300  or  400  cc  water,  and  heated  to  dissolve 
copper  sulphate. 

This  solution  is  treated  with  exactly  enough*  of  a  titrated 
solution  of  hydrochloric  acid  to  precipitate  the  silver,  and 
allowed  to  stand  twenty-four  hours,  after  which  the  precipi- 
tate (I.)  of  silver  chloride,  lead  sulphate,  antimony  oxide, 
etc.,  is  filtered  off  and  washed  with  water. 

The  filtrate  is  brought  to  a  volume  of  400-450  cc,  and 
the  copper  separated  by  electrolysis.  For  this  purpose, 
either  a  larger  platinum  dish  is  used,  or  the  platinum  cone 
shown  in  Fig.  61,  p.  88;  and  the  current  is  continued  only 
so  long  as  is  necessary  to  remove  the  copper,  as  otherwise 
it  might  be  contaminated  with  antimony  and  arsenic.  If 
the  copper  is  darkened  by  these  metals,  the  process  given 
on  p.  157  must  be  followed. 

There  is  usually  a  slight  deposit  of  lead  peroxide  on  the 
positive  electrode  which  is  determined  as  directed  p.  169. 
The  precipitated  copper  contains  bismuth.  To  determine 
the  latter,  the  copper  precipitated  from  both  25-g  portions 
is  dissolved  in  about  350  cc  nitric  acid,  sp.  gr.  1.2;  a  great 
excess  of  concentrated  hydrochlowc  acid  added,  and  the 
solution  boiled  till  all  the  nitric  acid  is  driven  off.  It  is 
evaporated  on  the  water-bath  till  the  residue  has  a  brown 

*  Silver  must  be  previously  determined  in  a  separate  portion  of  25  g1. 


APPENDIX.  271 


color,  and  then  poured  into  a  large  quantity  of  boiling  water 
to  separate  the  bismuth  as  oxychloride.  The  bismuth  oxy- 
chloride  is  generally  contaminated  with  some  basic  copper 
salt.  If  the  color  shows  the  quantity  of  the  latter  to  be 
considerable,  the  precipitate,  after  standing  twenty-four 
hours,  is  filtered  off,  dissolved  again  in  concentrated  nitric 
acid,  diluted  with  water,  and  copper  precipitated  electro- 
lytically  (p.  156). 

The  bismuth  in  the  solution  is  determined  gravimetrically. 

The  solution  siphoned  off  from  the  main  portion  of  the 
copper  is  evaporated  to  dryness,  and  the  sulphuric  acid  set 
free  by  the  precipitation  of  copper  removed  by  heating  on 
the  sand-bath,  so  that  the  residue  contains  only  traces  of  acid. 
After  cooling,  it  is  dissolved  in  hydrochloric  acid  and  water, 
any  silica  from  the  glass  vessels  filtered  off,  and  hydrogen 
sulphide  passed  into  the  solution,  heated  to  70°-80°,  till  it  is 
thoroughly  cool.  The  precipitate,  which  consists  mostly  of 
arsenic  and  antimony,  is  filtered  off  after  long  standing  on 
the  sand-bath,  and  washed  ;  the  filtrate,  containing  iron, 
cobalt,  etc.,  is  retained  (II.). 

Another  portion  of  the  antimony  is  in  residue  I.,  which 
was  left  on  the  solution  of  the  blister  copper  in  nitric  acid. 
Both  precipitates  are  digested  with  a  concentrated  solution 
of  sodium  sulphide,  filtered,  and  antimony  and  tin  determined 
as  directed  p.  222.  The  sulphides  insoluble  in  sodium  sulphide 
are  oxidized  with  nitric  acid,  silver  (p.  172),  and  lead  (p.  169) 
precipitated  from  the  solution,  the  solution  siphoned  off, 
evaporated  to  remove  nitric  acid,  and  bismuth  determined 
gravimetrically. 

Solution  II.  filtered  from  the  hydrogen  sulphide  precipi- 
tate, which  contains  iron,  cobalt,  etc.,  is  evaporated  to 
remove  hydrogen  sulphide,  etc.,  and  the  metals  are  deter- 
mined in  the  residue  as  directed  p.  267. 


272         QUANTITATIVE   ANALYSIS   BY   ELECTROLYSIS. 
REFINED     COPPER. 

This  contains,  in  addition  to  the  metals  present  in  blister 
copper,  cuprous  oxide.  The  metals  are  determined  as  in 
blister  copper.  The  determination  of  the  cuprous  oxide  is 
based  on  the  fact  that  it  reacts  with  a  dilute  neutral  silver 
solution,  with  the  formation  of  metallic  silver  and  basic 
copper  nitrate,  which  precipitate,  and  normal  copper  nitrate, 
which  remains  in  solution. 

3Cu2O  +  6AgNO3  +  3H2O 

=  2Cu2(OH)3N03  +  2Cu(N03)2  +  6Ag. 

The  process  is  as  follows:  About  2  g  silver  nitrate  is 
dissolved  in  100  cc  water,  and  about  1  g  of  the  copper  to 
be  tested  is  added.  When  the  reaction  is  ended  in  the  cold, 
the  precipitate  is  filtered  off,  and  washed  thoroughly  with 
water ;  either  the  copper  or  the  silver  in  it  may  be  deter- 
mined electrolytically.  The  nitric  acid  is  removed  by 
evaporation,  and  copper  and  silver  separated  as  directed 
p.  216.  If  copper  is  to  be  determined,  silver  is  precipitated 
as  silver  chloride  from  the  aqueous  solution  of  the  residue, 
the  excess  of  acid  removed,  and  copper  precipitated,  by 
electrolysis,  from  solution  of  copper  ammonium  oxalate 
(p.  155). 

TIN. 

The   Impurities   are   usually  Copper,  Lead,   Bismuth,  Iron,  Zinc, 
Arsenic,  and   Antimony. 

By  oxidation  of  the  metal  with  nitric  acid,  the  tin  is 
completely  converted  into  insoluble  oxide,  while  the  other 


APPENDIX.*  273 

metals  remain,  for  the  most  part,  in  solution.  The  tin  oxide 
contains,  however,  detenninable  quantities  of  lead,  copper, 
antimony,  and  arsenic.  The  methods  already  described  are 
used  for  their  separation  ;  the  tin  oxide  is  digested  with  a 
concentrated  solution  of  sodium  sulphide,  or  fused  with 
anhydrous  sodium  thiosulphite  in  a  porcelain  crucible. 

The  insoluble  sulphides  of  copper  and  lead  are  oxidized 
with  nitric  acid,  and  the  solution  added  to  the  principal 
solution  of  the  metals.  The  rest  of  the  process  is  in 
accordance  with  previous  directions. 

SILVER. 
Traces  of   Gold,  also  Lead,  Copper,  Antimony,  and  Arsenic. 

The  gold  remains  undissolved  when  a  large  quantity  of 
silver  is  dissolved  in  nitric  acid  entirely  free  from  hydro- 
chloric acid.  To  determine  copper  and  lead,  the  silver  is 
precipitated  from  the  largely  diluted  solution  by  hydro- 
chloric acid,  the  silver  chloride  filtered  off,  and  copper  and 
lead  separated,  after  removal  of  hydrochloric  acid,  as  directed 
p.  213. 

As  antimony  and  arsenic  can  only  be  present  in  very 
small  quantities,  they  are  determined  in  a  larger  weight  of 
silver.  The  silver  is  precipitated  as  chloride,  and  the  metals 
precipitable  by  hydrogen  sulphide  by  passing  the  gas  into 
the  hot  filtrate.  Antimony  and  arsenic  are  separated  from 
the  other  metals  by  digestion  with  sodium  sulphide,  and 
determined  as  usual  (p.  224). 


274        QUANTITATIVE  ANALYSIS  BY   ELECTROLYSIS. 

COMMERCIAL    NICKEL. 

Nickel,  Copper,  Arsenic,  Antimony,  Iron,  Cobalt  (Carbon, 
Silica,   Sulphur). 

The  nickel  is  dissolved  in  nitric  acid,  the  insoluble  residue 
filtered  off,  the  nitric  acid  removed  by  evaporation,  the  resi- 
due dissolved  in  hydrochloric  acid,  and  hydrogen  sulphide 
passed  in  to  remove  the  metals  which  it  will  precipitate.  It  is 
best  to  redissolve  the  sulphides  and  repeat  the  precipitation. 
Antimony  and  arsenic  are  separated  from  copper  by  digest- 
ing the  sulphide  with  sodium  sulphide,  and  determined  as 
usual*  It  is  to  be  noted,  in  determining  antimony,  that  the 
insoluble  residue  (silica,  etc.)  may  contain  antimony,  and 
must  be  tested  for  it. 

To  separate  cobalt  and  nickel  from  iron,  the  filtrate  from 
the  hydrogen  sulphide  precipitate  is  evaporated  to  dryness, 
the  residue  oxidized  with  hydrogen  peroxide  or  bromine 
water,  and  dissolved  in  water  with  addition  of  acetic  acid. 
The  metals  are  then  converted  into  double  oxalates  by 
addition  of  potassium  oxalate,  and  cobalt  and  nickel  precipi- 
tated by  a'cetic  acid.  The  two  metals,  and  the  iron  in  the 
filtrate,  are  determined  as  directed  p.  245. 

If  only  iron  is  to  be  determined,  the  three  metals  are 
precipitated  from  the  double  oxalate  solution  by  electrolysis, 
the  weight  ascertained,  and  the  iron  determined  volumetri- 
cally  in  hydrochloric  acid  solution  (pp.  191-193). 


APPENDIX.  275 

PIG    IRON,    STEEL,     SPIEGEL,    FERROMANGANESE. 

Constituents  :  Iron,  Manganese,  Copper,  Zinc,  Cobalt,  Nickel, 
Chromium,  Aluminium,  Titanium,  Arsenic,  Antimony,  Cal- 
cium, Magnesium,  Silicon,  Phosphorus,  Sulphur,  Carbon. 

If  a  complete  analysis  of  iron  is  to  be  made,  it  is  best  to 
dissolve  a  large  quantity,  dilute  to  a  known  volume,  and  use 
aliquot  parts  of  the  solution.  In  many  cases,  only  copper, 
or  manganese,  or  certain  other  metals  are  to  be  determined. 
The  complete  analysis  will  first  be  described,  and  afterward 
the  special  determination  of  certain  metals.  5  or  10  grams 
of  the  pure  iron,  in  powder  or  turnings,  are  dissolved  in 
hydrochloric  acid  in  a  capacious  platinum  or  porcelain  dish, 
and  the  solution  evaporated  to  dryness.  The  residue  is 
moistened  with  dilute  hydrochloric  acid,  allowed  to  stand  for 
a  time  that  the  acid  may  act,  dissolved  in  water,  and  the 
insoluble  residue  of  graphite,  silica,  and  compounds  of  iron 
with  titanium,  chromium,  phosphorus,  and  carbon,  filtered 
off.  The  precipitate  is  ignited  with  the  filter,  fused  with 
about  its  own  weight  of  a  mixture  of  equal  parts  of  sodium 
carbonate  and  potassium  nitrate,  dissolved  in  water  with 
addition  of  hydrochloric  acid,  and  the  solution  evaporated 
on  the  water-bath.  The  residue  is  heated  for  a  short  time  on 
the  sand-bath  to  insure  separation  of  silica,  moistened,  after 
cooling,  with  hydrochloric  acid,  treated  with  water,  heated, 
and  the  silica  filtered  off,  weighed,  and  tested  for  titanium. 
The  filtrate  contains  chromium,  together  with  the  rest  of 
the  silica  and  titanium,  and  small  quantities  of  iron  and 
aluminium.  To  completely  separate  silica  and  titanium,  the 
solution  is  evaporated  to  dryness,  the  residue  treated  with 


276         QUANTITATIVE   ANALYSIS   BY    ELECTROLYSIS. 

dilute  sulphuric  acid,  and  heated  till  all  the  hydrochloric 
acid  is  driven  off;  water  is  then  added,  silica  filtered  off, 
and  titanic  acid  precipitated  by  long  boiling.  The  filtrate 
from  titanic  acid  is  concentrated  by  evaporation,  the  free 
sulphuric  acid  neutralized  with  ammonia,  iron,  aluminium, 
and  chromium  converted  into  the  double  oxalates,  and 
chromium  separated  as  directed  p.  153. 

For  the  determination  of  iron,  aluminium,  zinc,  cobalt, 
nickel,  manganese,  copper,  calcium,  and  magnesium,  an  aliquot 
part  of  the  hydrochloric  acid  solution  is  saturated  with 
hydrogen  sulphide,  and  the  precipitate  filtered  off  after  long 
standing  in  a  warm  place. 

Since  arsenic  and  antimony  are  ordinarily  present  only  in 
very  small  quantities,  the  copper  sulphide  can  usually  be 
oxidized  with  nitric  acid,  and  the  copper  determined  electro- 
lytically.  If  the  precipitated  copper  is  blackened  by  the 
presence  of  antimony  or  arsenic,  it  is  treated  as  directed 
p.  157. 

The  filtrate  from  the  hydrogen  sulphide  precipitate  is 
freed  from  hydrogen  sulphide  and  hydrochloric  acid  by 
evaporation,  oxidized  with  hydrogen  peroxide  or  a  little 
bromine  water  (by  no  means  with  nitric  acid),  dissolved  in 
water  with  addition  of  a  little  acetic  acid,  and  the  metals 
converted  into  double  oxalates  by  the  use  of  potassium  (not 
ammonium)  oxalate.  The  insoluble  calcium  oxalate  is  filtered 
off,  and  separated  from  the  manganese  precipitated  with  it  as 
directed  p.  239.  The  filtrate  is  diluted  *  with  water,  heated 
to  boiling,  and  an  excess  of  concentrated  acetic  acid  added, 
whereby  all  the  zinc,  cobalt,  nickel,  and  magnesium,  and  a 
portion  of  the  manganese,  are  precipitated  as  oxalates ;  iron, 
aluminium,  and  the  rest  of  the  manganese  remain  in  solution 

*  Fifty  cc  of  the  dilute  solution  should  contain  0.4-0.5  g  iron. 


APPENDIX.  277 

as  double  oxalates.  The  beaker  is  covered,  and  left  standing 
in  a  warm  place  for  six  hours ;  the  precipitate  is  then  filtered 
off,  washed  with  a  mixture  of  equal  volumes  of  acetic  acid, 
water,  and  alcohol,  and  dissolved,  after  drying,  in  ammonium 
oxalate.  Zinc,  cobalt,  and  nickel  are  separated  from  man- 
ganese and  magnesium  as  directed  p.  251.  The  filtrate  from 
the  oxalates  is  completely  freed  from  alcohol  and  acetic  acid  by 
evaporation,  and  iron,  aluminium,  and  manganese  separated 
as  directed  pp.  194-198.  As  the  quantity  of  zinc,  cobalt, 
etc. ,  is  generally  very  small,  it  is  best,  in  order  to  facilitate  the 
separation  of  the  oxalates  and  the  collection  of  the  precipi- 
tate, to  add  about  0.2  g  magnesium  in  the  form  of  chloride,* 
so  that  magnesium  oxalate  is  precipitated  with  the  other 
oxalates.  In  this  case,  the  magnesium  in  pig  iron,  if  present 
at  all,  is  determined  in  another  portion,  together  with  some 
other  metal  (e.g.,  copper).  If  magnesium  is  used,  all  the 
manganese  is  found  in  the  precipitate  produced  by  acetic  acid. 
To  determine  manganese  alone  in  pig  iron,  either  an 
aliquot  part  of  the  hydrochloric  acid  solution,  or  a  separate 
portion  of  0.2-0.5  g  iron  may  be  taken,  and  the  determina- 
tion conducted  as  directed  under  Spathic  Iron  Ore,  p.  239. 
If  copper  is  to  be  determined,  the  solution  freed  from  acid 
and  preferably  oxidized  is  treated  with  ammonium  oxalate 
in  great  excess,  and  electrolyzed  as  already  directed.  The 
hydrochloric  acid  solution  may  also  be  precipitated  with 
hydrogen  sulphide,  and  the  copper  determined  in  nitric  acid 
solution  (see  p.  156). 

Determination  of  Arsenic  and  Antimony. 

Since  these  metals  are  present  only  in  very  small  quantity, 
about  10  g  pig  iron  are  used  for  their  determination,  and 

*  Dissolve  magnesium  oxide  in  hydrochloric  acid,  and  remove  the  free 
acid  by  evaporation. 


278         QUANTITATIVE   ANALYSIS   BY    ELECTROLYSIS. 

digested  with  aqua  regia.  When  solution  is  complete,  the 
aqua  regia  is  removed  by  evaporation,  the  residue  treated 
with  hydrochloric  acid,  and  heated  till  no  nitric  acid  remains. 
The  solution  is  diluted,  heated,  and  hydrogen  sulphide 
passed  into  it  until  it  is  thoroughly  cool  (p.  253) ;  the  precipi- 
tated sulphides  of  arsenic,  antimony,  and  copper  are  filtered 
off,  thoroughly  washed,  and  digested  with  sodium  sulphide. 
The  solution  is  treated  like  one  containing  polysulphides, 
and  arsenic  and  antimony  separated  as  directed  p.  224. 

Determination  of  Phosphorus. 

About  2  g  of  iron  is  digested  with  nitric  acid,  sp.  gr. 
1.2,  till  decomposition  is  complete.  If  a  carbonaceous  residue 
is  left,  the  nitric  acid  solution  is  poured  off,  and  the  residue 
heated  with  aqua  regia.  Nitric  acid  and  aqua  regia  are 
completely  removed  by  evaporation  to  dryness,  and  the 
nitrates  converted  into  chlorides  by  repeatedly  moistening 
with  concentrated  hydrochloric  acid,  and  evaporating  to 
dryness.  '  The  residue  is  treated  with  water,  heated,  and 
the  iron  brought  into  solution  by  the  addition  of  the  least 
possible  quantity  of  hydrochloric  acid.  To  convert  the  iron, 
etc.,  into  double  oxalates,  six  or  eight  times  the  weight  of 
the  iron,  reckoned  as  oxide,  of  a  mixture  of  1  part  potassium 
oxalate  and  5-6  parts  ammonium  oxalate,  is  dissolved  by 
heating  in  the  solution,  it  is  diluted  to  250-300  cc,  and 
electrolyzed  at  a  temperature  of  about  80°,  The  heating 
is  maintained  during  the  reaction ;  the  solution  must  by  no 
means  be  heated  to  boiling,  lest  the  iron  scale  off.  The  solu- 
tion is  poured  off  when  the  reduction  is  complete,  and  phos- 
phoric acid  determined  as  magnesium  pyrophosphate. 

Two  grams  of  iron  are  enough  for  tiie  determination  of 


APPENDIX.  279 

phosphorus,  even  when  the  percentage  is  small.  If  a  larger 
quantity  is  taken,  it  is  best  to  divide  the  solution,  after 
conversion  into  oxalates,  and  precipitate  in  several  dishes. 
As  it  is  not  necessary  to  determine  the  iron,  it  may  be 
precipitated  just  as  well  in  a  beaker;  in  this  case,  the 
negative  electrode  is  a  large  piece  of  light  platinum  foil 
which  is  attached  by  a  platinum  wire  to  the  negative  pole  of 
the  source  of  current. 

Determination   of  Sulphur. 

About  2  grams  of  iron  is  oxidized,  with  aqua  regia,  to 
convert  sulphur  into  sulphuric  acid,  and  the  insoluble  resi- 
due filtered  off.  As  a  portion  of  the  sulphur  may  be  left 
in  the  residue,  it  is  fused  with  a  small  quantity  of  a  mixture 
of  sodium  carbonate  and  potassium  nitrate,  the  fused  mass 
dissolved  in  hydrochloric  acid,  and  the  solution  thus  obtained 
added  to  the  other.  The  aqua  regia  is  removed,  the  nitrates 
converted  into  chlorides,  and  the  latter  into  double  oxalates, 
as  already  directed.  After  removing  the  iron  by  electrolysis, 
the  solution  is  poured  off,  boiled  to  remove  ammonia,  acidified 
with  hydrochloric  acid,  and  the  sulphuric  acid  precipitated 
with  barium  chloride. 


280        QUANTITATIVE   ANALYSIS   BY   ELECTROLYSIS. 


TABLES     FOR    CALCULATION    OF    ANALYSES, 


A  t.ATVI  1  f> 

Weight. 

Found. 

Required. 

Factor. 

Aluminium    . 

27.04 

A1A 

Al 

0.5304 

Antimony      .     . 

119.6 

Sb 

SbaO. 

1.20017 

Sb2S3 

1.40108 

Arsenic     .     .     . 

74.9 

As 

AsA 

1.31962 

AsA 

1.53271 

As.2S3 

1.64192 

Barium 

136.86 

BaSO4 

Ba 

0.58819 

Ba003 

Ba 

0.69574 

|     BaO 

0.77688 

Beryllium      .     .            9.08 

BeO 

Be 

0.36262 

Bismuth    .     .     .         208.4 

Bi 

BiA 

1.11488 

Boron  .     .     .     .           10.9 

KBF4 

B 

0.08639 

BA 

0.27613 

Bromine!   .     .     .           79.76 

AgBr 

Br 

0.42556 

Cadmium  .     .     .         111.7 

Cd 

CdO 

1.14288 

CdS 

1.28630 

Caesium     . 

132.7 

Calcium    . 

39.91 

CaO 

Ca 

0.71433 

CaCO8 

Ca 

0.40006 

CaO 

0.56004 

Carbon 

11.97 

CO2 

C 

0.272727 

Ca003 

C02 

0.43995 

Cerium      .     .     . 

141.2 

Chlorine   . 

35.37 

AgCl 

Cl 

0.24729 

Ag 

Cl 

0.32853 

Chromium     .     . 

52.0 

CrA 

Cr 

0.81419 

CrO3 

1.18581 

1 

TABLES   FOR   CALCULATION   OF   ANALYSES. 


281 


Atomic 
Weight. 

Found. 

Required. 

Factor 

Cobalt      .    .     . 

58.60 

Co 

CoO 

L27116 

Copper     .    .     . 

63.18 

Cu 

CuO 

1.25261 

CuS 

Ic25309 

Diclymium     .     . 

145.0 

Erbium     . 

166.0 

Fluorine  . 

19.06 

CaF2 

F 

0.48853 

Gold    .... 

196.7 

Au 

Aa2Oe 

1.12171 

Hydrogen 

1 

H20 

H 

0,11136 

Iodine.     . 

126.54 

Agl 

I 

0.54031 

Ag 

I 

1.17546 

Iron     .... 

55.88 

Fe 

FeO 

1.28561 

Fe208 

1.42842 

Lanthanum   .     . 

138.5 

Lead    .... 

206.39 

PbO2 

Pb 

0.86605 

PbO 

0.93303 

PbCl2 

1.16289 

Lithium    .     .     . 

7.01 

LiCl 

Li 

0.165408 

Li20 

0.35370 

Li3P04 

Li 

0.18156 

Li2O 

0.38824 

LiCl 

1.09764 

Magnesium   . 

23.94 

Mg2P207 

Mg 

0,21614 

MgO 

0.36024 

Manganese    .     . 

54.8 

Mn304 

Mn 

0.72029 

MnO 

0.93007 

- 

Mn2O8 

1.03496 

Mn02 

Mn 

0.63192 

MuO 

0.81596 

Mn2O8 

0.90798 

MnS04 

Mn 

0.36383 

MnO 

0.46979 

Mn2O8 

0.52277 

282        QUANTITATIVE  ANALYSIS  BY   ELECTEOLYSIS. 


Atomic 
Weight. 

Found. 

Required. 

Factor. 

Mercury    .     .     . 

199.8 

Hg 

Hg20 

1.03994 

HgO 

1.07988 

HgCl 

1.17703 

Hg2S 

1.08003 

HgS 

1.16006 

Molybdenum 

95.9 

MoS8 

Mo 

0.49989 

Nickel.     .     .     . 

58.6 

Ni 

NiO 

1.27116 

Niobium  .     .     . 

93.7 

Nitrogen  . 

14.01 

Pt 

N 

0.14411 

NH3 

0.17497 

NH4 

0.18526 

Osmium    . 

195 

Palladium      .     . 

106.2 

Phosphorus    . 

30.96 

Mg2P2O7 

P 

0.27952 

P2O5 

0.63976 

Platinum  .     .     . 

194.43 

Pt 

Pt02 

1.16417 

Potassium      .     . 

39.03 

Pt 

K 

0.40129 

K2O 

0.48848 

.  1  ". 

KC1 

0.76495 

K2S04 

0.89389 

Rhodium  .     .     . 

104.1 

Rubidium 

85.2 

Ruthenium     .     . 

103.5 

Selenium  . 

78.87 

Silicon 

28 

Si02 

Si 

0.46729 

Silver  .... 

107.66 

Ag 

Ag20 

1.07412 

AgCl 

1.32853 

Sodium     , 

22.99 

NaCl 

Na 

0.39393 

Na2O 

0.53067 

Na2SO4 

1.21488 

Strontium 

87.3 

SrS04 

Sr 

0.47673 

SrO 

0.56389 

TABLES   FOR   CALCULATION    OF  ANALYSES. 


263 


Atomic 
Weight. 

Found. 

Required. 

Factor. 

Sulphur    .     .     . 

31.98 

BaSO4 

S 

0.13744 

S08 

0.34322 

S04 

0.41181 

Tantalum       .     . 

182 

Tellurium      .     . 

127.7 

Thallium  . 

203.7 

T12O 

Tl 

0.9623 

Thorium  . 

231.96 

Tin       .... 

118.8 

Sn 

SnO2 

1.26869 

Titanium  .     .     . 

50.25 

Ti02 

Ti 

0.61154 

Tungsten  . 

183.6 

W08 

W 

0.79316 

Uranium  . 

239.8 

UO2 

U 

0.88249 

U808 

1.03916 

Vanadium 

51.1 

V2O5 

V 

0.56154 

Yttrium    . 

89.6 

Zinc     .... 

65.1 

Zn 

ZnO 

1.24516 

ZnS 

1.49124 

Zircon 

90.4 

Zr02 

Zr 

0.73904 

284        QUANTITATIVE  ANALYSIS   BY   ELECTROLYSIS. 


REAGENTS. 


POTASSIUM    OXALATE. 

The  crystallized  potassium  oxalate  of  commerce  always 
contains  determinable  quantities  of  iron  and  lead.  To  purify 
it,  one  part  of  the  salt  is  dissolved  in  three  parts  of  water  in 
a  porcelain  dish,  and  ammonium  sulphide  is  added  drop  by 
drop,  as  long  as  a  precipitate  forms.  The  solution  is  now 
heated  on  the  water-bath  till  the  precipitate  settles,  and 
filtered  through  a  plaited  filter.  To  decompose  the  slight 
excess  of  ammonium  sulphide,  a  current  of  air  is  conducted 
through  the  solution  till  it  is  perfectly  colorless,  and  no 
longer  gives  a  reaction  with  sodium  nitroprusside.  The 
separated  sulphur  is  allowed  to  settle,  and  the  clear  solution 
siphoned  off. 

AMMONIUM     OXALATB. 

The  same  impurities  are  present  as  in  potassium  oxalate. 
The  salt  is  purified  by  precipitating  the  hot  saturated  solution 
with  ammonium  sulphide.  It  is  heated  over  a  naked  flame  till 
the  precipitate  coheres  together,  and  filtered  hot  by  the  use 
of  a  hot-water  funnel.  The  greater  part  of  the  ammonium 
oxalate  crystallizes  from  the  filtrate  on  cooling.  The  solution 
is  poured  off,  and  the  crystals  dried  by  placing  them  in  a 
funnel  stopped  with  asbestos,  and  connecting  with  a  filter- 
pump. 


REAGENTS.  285 

OXALIC    ACID. 

The  impurities  are  similar  to  those  of  the  alkali  oxalates ; 
it  is  purified  by  repeated  recrystallization. 

AMMONIUM    SULPHATE. 

This  salt  is  purified  in  the  same  way  as  ammonium 
oxalate. 

SODIUM    SULPHIDE. 

The  crystallized  sodium  sulphide  of  commerce  is  not  only 
exceedingly  impure,  but  is  not  inonosulphide  at  all,  but 
a  mixture  of  polysulphides  and  sodium  hydroxide.  The 
presence  of  the  latter  explains  that  of  alumina,  which  is 
always  found  in  abundance.  If  commercial  sodium  sulphide 
is  used,  its  solution  must  first  be  completely  saturated,  without 
access  of  air,  with  hydrogen  sulphide  gas.  It  is  better, 
however,  to  prepare  the  substance,  in  which  case  the  process 
is  as  follows  :  Sodium  hydroxide  purified  by  alcohol  is 
dissolved  in  water  to  a  solution  of  sp.  gr.  1.35.  The  solution 
is  divided  into  two  equal  parts,  and  one  half,  with  exclusion 
of  air,  saturated  with  the  purest  possible  hydrogen  sulphide 
gas  till  the  volume  ceases  to  increase.  The  hydrogen  sul- 
phide is  purified  by  passing  it  through  a  wash-bottle  of  water, 
and  several  tubes  filled  with  cotton  or  wadding.  When 
completely  saturated,  the  solution  is  filtered  from  the  pre- 
cipitate formed,  and  mixed  with  the  other  half  of  the, sodium 
hydroxide  solution.  Hydrogen  sulphide  is  again  passed  into 
the  mixture,  with  exclusion  of  air,  and  it  is  filtered  again. 
The  nearly  colorless  filtrate  is  evaporated  in  a  capacious 
platinum  or  porcelain  dish,  over  a  strong  free  flame  as  quickly 
as  possible.  It  boils  without  bumping  if  a  platinum  spiral  is 


QUANTITATIVE  ANALYSIS   BY   ELECTROLYSIS. 

placed  in  it.  As  soon  as  a  thin  crystalline  pellicle  forms  on 
the  surface,  the  boiling  is  stopped,  and  the  solution  poured 
while  hot  into  small  flasks  with  well-ground  glass  stoppers 
which  must  be  filled  full.  It  is  best  to  completely  exclude 
the  air  by  melted  paraffine.  For  the  separation  of  antimony 
and  tin,  the  solution  should  have  a  sp.  gr.  of  1.22-1.225. 

ALCOHOL. 

The  alcohol  used  for  washing  metals  must  be  free  from 
acid,  and,  as  nearly  as  possible,  absolute.  It  is  left  standing 
in  a  large  flask,  for  twelve  hours,  over  quicklime,  and  then 
distilled  off  on  a  water  or  steam  bath.  The  distillate  must 
leave  no  residue  on  evaporation. 


INDEX  OF  AUTHORS. 


PAGE 

Andrews  and  Campbell 143,  144 

Arrhenius 6,  9 

Bauer  and  Classen 187 

Becquerel 102 

Bergmaun 104 

and  Fresenius 141,  142,  143,  144,  172,  174 

Beilstein 104 

and  Jawein 145,163,165 

Blake  and  Chittendeu 181 

Bloxam 102 

Boisbaudran 104-153 

Bongartz  and  Classen 183 

Brand 137,  141,  143,  145,  148,  162,  174 

Brugnatelli 177 

Bunseu 45 

Campbell  and  Andrews. .. 143,  144 

Cheney  and  Richards 141,  143 

Chittenden  and  Blake 181 

Clamoud  64 

Clarke,  F.  W 104,  163,  174 

Classen 105,  135,  137,  141,  143,  148,  153,  154,  163,  166,  167,  178,  182, 

183,  191,  192,  194,  196,  199,  200,  201,  202,  205,  206,  213,  221,  224,  225, 
2-27,  228,  239,  245,  255,  256 

Classen  and  Bauer 187 

and  Bongartz 183 

"       and  Lud wig ,  174,  225 

"       and  v.  Reiss,  137,  141,  143, 145, 148,  154,  162,  163,  168,  178,  183,  188 

Cozzi 102 

Croasdule  154 

Cruikshauk 102 

287 


288  INDEX   OF   AUTHORS. 

PAGk. 

Daniell 42 

Danueel 70 

Despretz 102 

Dolezalek 38 

Drown  and  Mackeima. 137 

Duprfc 177 

Eiseiiberg 147,  164,  172,  174 

Elbs 47,  71 

Eliasberg 162,  164,  209 

Eugels 96,  148,  150,  151,  159,  183,  186,  211 

Faraday 7,  10,  12 

Farbaky  49 

Fischer 102 

Fischer-Hufschmidt 226 

Foote 154 

Fraukl 174 

"      and  Smith 148 

Fresenius 104 

andBergmami 141,  142,  143,  144,  172,  174 

Freudeuberg 17,  106,  183,  212,  215,  216,  217,  218,  220,  221 

Gaultier 102 

Gibbs 103,  106,  141,  143,  145,  153,  183 

Gobbels 182 

Groeger.... 148 

Grove  44 

Gillcher...;. 69 

Hampe 154,  166,  270 

Haunay 104 

v.  Helmholtz 12 

Heidenreich 105,  137,  140,  154,  158,  163,  166,  176,  183,  185,  207,  208, 

210,  212,  215,  216,  218 

Herpiii 92,  153 

Hofer 100 

Hoskinson 174 

Ikle 106 

"   and  Reinhardt 145 

Jannasch * 211 

Jawein 104 

and  Beilslein 145,163,165 


INDEX   OF   AUTHORS.  289 

PAGE 

Jordis 145,147 

Knufmann   134 

Kiliani 17,  106,  166 

"      and  v.  Miller 147 

Kinnicutt 172 

v.  Elobukow 97,  99 

Knerr  aud  Smith 162,  174,  183,  209 

Koliu  and  Woodgate 141,  143 

Krcichgauer 167 

Kriiger 107 

Ki  utvvig 1 73 

Le  Blanc 15,  106 

Leclanche 40 

Lecreuier 178 

Leuher  and  Rising , . . . .  174 

Le  Roy 141 ,  143 

Lob 134 

Luckow 103-106,    137,  141,  143,  145,  148,154,  155,  162-164,  166,  168, 

172,  177,  178,  182,  183,  188.  189,  202 

Ludwig 226 

aud  Classen 174,225 

Mackintosh 154 

v.  Malapert •. . .   89,  91 

Mascazziui  and  Parodi 104,  144,  166,  178 

Meckenna  and  Drown 137 

Medicus 167 

Meeker 154 

Meidiuger . .- 41 

Merrick 141,  143,  153 

v.  Miller  and  Kiliani 147 

Millot 145 

Moore 137,  148,  162,  163,  188 

Morton: 102 

Moyeraud  Smith 162,  205,  212,  218 

Muhrand  Smith 137,  177,  228 

Xernst 38 

Neumann 167,  170,  284,  235,  237 

andNissenson 213,  215,  219 

Nickles..  .  102 


290  INDEX    OF  AUTHORS. 

PAGE 

Nissenson  and  Neumann 213  215  219 

Noe '  66 

Oettel 55,  141-144,  154,  158 

Obi 141,  143,  153 

Ohm 12 

Ostwald 35 

Paget 71 

Parodi  and  Mascazzini 104,  144,  166,  178 

Persoz 177 

Regelsberger 154,  159 

Reinhardt 106 

"         and  Ihle 145 

v.  Reiss  and  Classen....  137,   141,  143,  145,  148,  154,  162,  163,  168,  178, 

183,  188 

Richards  and  Cheney. 141,  143 

Riche 92,  141,  143,  145,  148,  154,  166 

Richert 104 

Rising  arid  Lenher 174 

Rudorff....  137,   141,  143,  145,  148,  154,  157,  162,  166,  172,  174,  177,  178, 

182,  183,  205,  207,  208 

Saltar  and  Smith 162 

Schelle,  R 50 

Scheneck 49 

Schmucker 162,  217 

Schroder 156 

Schucht 141,  143,  148,  162.  166,  172,  183 

Schweder 141,  143,  154 

Smith,  E.  F. ...  79,  105,  137,  140,  154,  158,  163-166,  174,  176,177,182, 

183,  207,  209,  216,  228,  243 

and  Frankel 145,  174 

"     Knerr 162,174,183,209 

"    Moyer 162,  205,  212,  218 

"    Muhr 137,177,228 

"     Saltar 162 

"          "    Thomas 162 

"    Wallace 177,  204,  205.  208,  210,  212,  215,  228 

Tenny..'. 166 

Thomas  and  Smith 162 

Thomson,  W 37 


INDEX   OF  AUTHORS.  291 

PAGE 

van't  Hoff 6 

Vortraaun....  106,  137,  141,  143,  145,  162,  163,  165-167,  174,  178,  180-190, 

192-194,  202,  204,  234 

Wallace  and  Smith 177,  204,  205,  208,  210,  212,  215,  228 

v.  Waltenhofeu 68 

Warwick 145,  154,  163,  167,  205 

Wirkner 177 

Wohler 102,  183 

Woodgate  and  Kohn 141,  143 

Wrightson 103,  137,  141,  143,  144,  153,  163,178 


104,  209 


INDEX    OF    SUBJECTS. 


PAGE 

Accumulators 47 

action  of 48 

charging  of 56 

general  rules  for  the  handling  of 54 

tests  of 51 

Acids,  decomposition  tension  value  for 16 

dissociation  of 8 

Alcohol  as  reagent 286 

Alloy,  analysis  of,  containing  antimony  and  arsenic 238 

antimony  and  tin 238 

antimony,  tin,  and  arsenic 239 

antimony  and  lead 237 

copper  and  nickel 233 

copper  and  silver 233 

copper  and  tin 235 

copper,  tin,  zinc,  and  phosphorus 235 

copper,  tin,  zinc,  manganese,  and  phos- 
phorus    236 

copper  and  zinc 231 

copper,  zinc,  and  nickel 234 

tin  and  lead 236 

tin,  lead,  bismuth,  and  cadmium  237 

Aluminium,  determination  of 153 

separation  from  cobalt 204 

iron 196 

iron  and  beryllium 201 

iron  and  chromium ......  200 

nickel 206 

zinc 208 

Ammonium,  determination  of.    188 

separation  from  sodium 229 

293 


294  INDEX    OF   SUBJECTS. 

PAGE 

Ammonium,  oxalate  as  reagent 284 

sulphate  as  reagent  285 

Ampere,  definition  of 12 

Amperemeter 31 

Analysis,  arrangements  for 107  ff 

process  of 83 

Anions 1?  8 

Antimony,  determination  of 178 

separation  from  arsenic 224 

arsenic  and  tin 225 

lead 219 

mercury 220 

silver 220 

tin 221 

glance,  analysis  of 259 

metallic,  analysis  of 268 

Arsenic,  determination  of 188 

separation  from  antimony 224 

antimony  and  tin 225 

mercury 220 

silver 220 

Arsenopyrite,  analysis  of 253 

Bases,  decomposition  tension  value  for 16 

dissociation  of 8 

Beryllium,  determination  of 153 

separation  from  aluminium 201 

iron 201 

Bismuth,  determination  of 162 

separation  from  cobalt. 205 

Bog-iron  ore,  analysis  of 242 

Bournonite,  analysis  of 260 

Brass,  analysis  of 231 

Bromine,  determination  of 190 

Bronze,  analysis  of 235 

Buusen  cell 45 

Cadmium,  determination  of 163 

separation  from  copper 212 

lead 217 

manganese 212 

mercury. 218 

zinc..  .  209 


INDEX   OF   SUBJECTS.  295 

PAGE 

Calamiue,  analysis  of 249 

Calculation  of  analyses,  tables  for 280-2S3 

Cathions , 1,  8 

Cerussite,  analysis  of 263 

Chalcopyrite,  analysis  of 254 

Chlorine,  determination  of 190 

Chrome-iron  ore,  analysis  of 242 

Chromium,  determination  of 153 

separation  from  iron 199 

iron  and  aluminium 200 

iron  and  uranium 200 

iron,  uranium,  and  cobalt 204 

iron  and  nickel 206 

Cinnabar,  analysis  of 265 

Clay-iron  ore,  analysis  of 242 

Cobalt,  determination  of 141 

separation  from  aluminium 204 

bismuth 205 

chromium 204 

chromium  and  uranium 204 

copper ..205 

iron 191 

lead 206 

mercury 206 

uranium 204 

zinc 204 

Cobaltiferous  arsenopyrite,  analysis  of 262 

Cobaltite,  analysis  of 261 

Conductivity  of  solutions,  theory  of 20 

Copper,  determination  of 153 

separation  from  antimony  and  arsenic 157 

arsenic 217 

cadmium 212 

cobalt 205 

lead  213 

manganese 211 

mercury 217 

nickel 206 

silver 215 

zinc 206 

Copper,  blister,  analysis  of 269 

matte,  analysis  of 215,  255 


296  INDEX   OF   SUBJECTS. 

PAGE 

Copper,  refined,  analysis  of 272 

speiss,  analysis  of 255 

Cupron  element 46 

Current  density,  calculation  of 18 

specific  directions  concerning 139 

distribution,  scheme  of 126 

strength,  measurement  of  17,  28 

during  analysis 109,  122,  132 

apparatus  for  regulation  of 73,  75 

Daniell  cell 42 

Decomposition,  tension  value  of,  for  acids  and  bases 15 

Double  oxalates,  general  advantages  of,  for  quantitative  analysis 5 

Dynamo,  action  of 62 

Edison-Lalaude  element , 47 

Electrochemical  equivalent 12 

Institute  at  Aachen,  former  equipment  of Ill 

present        "  " 124 

Electrodes 84,  85,  88,  89,  92-94 

Electrode  tension 13 

measurement  of 110,  124 

Electrolysis,  influence  of  temperature  on 94,  95 

special  apparatus  for 100,  101 

Electrolytic  dissociation 6 

of  acids 8 

bases 8 

salts 8 

Electrolytic  dissociation ,  degree  of 9,  20 

precipitation,  theory  of 21 

solution  pressure 14 

Electrometers 35-37 

Faraday's  law 10 

Ferromangauese,  analysis  of 275 

Furnace  "sows,"  analysis  of 258 

Galena,  analysis  of 263 

Galvanometers  29 

German  silver,  analysis  of •  •  •  234 

Gold,  determination  of 1?7 

separation  from  other  metals 228 

Gravity  cell 43 

Grove  cell.. 44 


INDEX   OF   SUBJECTS.  297 

PAGE 

Halogens,  determination  of 190 

precipitation  of 22 

Heating,  arrangements  for 95,  96 

Hematite,  analysis  of , , 240 

High  tension,  laboratory  for  experiments  with 133 

Historical 101 

Iodine,  determination  of , . .  190 

Ions 7 

"     osmotic  pressure  of 13 

Ion  theory 6 

Iridium,  separation  from  platinum 228 

Iron,  determination  of 137 

separation  from  aluminium 196 

aluminium  and  beryllium 201 

aluminium  and  chromium 200 

beryllium , 201 

chromium 199 

chromium  and  uranium 200 

cobalt 191 

copper 202 

lead 204 

manganese 194 

nickel. ...   192 

uranium  198 

zinc 193 

Laboratory,  private 12? 

for  the  electro-analysis  of  metals 130 

for  experiments  with  high  and  low  tensions 133 

Lead,  determination  of 166 

separation  from  antimony 219 

cadmium 217 

cobalt 206 

copper 213 

iron 204 

mercury 218 

silver ' 218 

zinc 210 

crude,  analysis  of 265 

hard,         ••        " 219,237 

matte,        "        " 264 

soft,  "         " .265 


298  INDEX   OF   SUBJECTS. 

PAGE 

Lead,  speiss,  analysis  of 256 

Leclanche  cell 39 

Lecture- room 130 

Limonite,  analysis  of 241 

Linnseite,          «        " 261 

Lippmann  electrometer 35 

Manganese,  determination  of 148 

separation  from  cadmium 212 

copper 211 

iron 194 

nickel 206 

zinc 208 

phosphor-bronze,  analysis  of 236 

Meidinger  cell 41 

Mercury,  determination  of 174 

separation  from  antimony 220 

cadmium 218 

copper 217 

lead 218 

nickel 207 

zinc 210 

Nickel,  determination  of 143 

separation  from  aluminium 206 

chromium 206 

copper 206 

iron 192 

lead 207 

manganese 206 

mercury 206 

uranium 206 

coin,  analysis  of 233 

commercial,  analysis  of  274 

matte,                  "        " 255 

Nitric  acid,  electrolysis  of 3 

in  nitrates,  determination  of 189 

Ohm 12 

Ohm's  law 12 

Organic  compounds,  electrolysis  of 4 

Osmotic  pressure  of  the  ions 13 

Oxalic  acid,  as  reagent  285 

Oxyhydrogen  gas  voltameter 24 


INDEX   OF   SUBJECTS.  299 

PAGE 

Palladium,  determination  of 183 

Peroxides,  precipitation  of 22 

Phosphor-bronze 235 

Phosphoric  acid,  separation  from  tin 228 

Pig  iron,  analysis  of 275 

Platinum  dishes  as  electrodes 84 

determination  of 182 

separation  from  iridiuin 228 

Poggendorf  compensation  method 35 

Polarization  current,  explanation  of 14 

Potassium  oxalate,  as  reagent 284 

sulphate,  electrolysis  of 4 

determination  of 188 

separation  from  sodium 229 

Psilomelane,  analysis  of 244 

Pyrargyrite,       "        " 257 

Pyromorphite,    "        " 264 

Quadrant  electrometer 37 

Reagents,  preparation  of 284-286 

Resistance  for  high  tensions 134 

roll 122 

significance  of , 19 

wire-gauze 113 

Rheostats 81 

Salts,  decomposition  tension  value  of 15 

Secondary  elements  (see  Accumulators) 47 

reactions 3,  4 

Separation  of  metals,  directions  for  the 191 

Shunt  circuit,  theory  of 74 

Silver,  determination  of 172 

separation  from  antimony 220 

arsenic 220 

copper 215 

lead 218 

zinc 210 

coin,  analysis  of. ...  233 

commercial  metal,  analysis  of 273 

Sine  galvanometer 28 

Smithsonite,  analysis  of 249 

Slag,  blast-furnace,  cupola,  and  bessemer 252 


300  INDEX   OF   SUBJECTS. 

PAGE 

Slag,  copper  and  lead 250 

Slags,  refinery 250 

Sodium,  separation  from  ammonium 229 

potassium 229 

sulphide  as  reagent 285 

Solder,  analysis  of 236 

Solutions,  requirements  of,  for  quantitative  electrolysis 5 

Spathic  iron-ore,  analysis  of 239 

Spelter,                       "        " 268 

Sphalerite,                  "        " 247 

Spiegel,                        "         " 275 

Spring  galvanometer 31 

Standards  for  electrolysis 86,  87 

v.  Klobukow's  universal 98 

Steel ,  analysis  of 275 

Stibnite,  "        " 259 

Storage-batteries  (see  Accumulators). 

Tables  for  calculation  of  analyses 280 

Tangent  galvanometer 26 

Tension 1*2 

measurement  of 32,  109,  124,  128,  182 

Tetrahedrite,  analysis  of .- 257 

Thallium,  determination  of 170 

Thermopiles 64 

Clamoud's 64 

Noe's 66 

Gulcher's 69 

Paget's 71 

regulator  for 70 

Tin,  determination  of 183 

separation  from  antimony 221 

arsenic 225 

phosphoric  acid 228 

analysis  of  commercial 272 

Torsion  galvanometer 33 

Transformer,  direct-current 125 

Transformation  of  current 125 

Type  metal,  analysis  of 219,  237 

Ullmaiiite,  analysis  of 259 

Ultramarine,    "         " 249 

Uranium,  determination  of 153 


INDEX   OF   SUBJECTS.  301 

PAGE 

Uranium,  separation  from  cobalt 204 

cobalt  and  chromium 204 

iron 198 

iron  ami  chromium 200 

nickel 206 

Volt 12 

Voltameter,  oxy hydrogen  gas 24 

weight 26 

Voltmeter 32 

Wood's  metal,  analysis  of 237 

Zinc  blende,  analysis  of 247 

crude,          "         " 268 

determination  of 144 

separation  from  alumin  iuin  208 

cadmium 209 

cobalt 204 

copper 208 

iron 193 

lead 210 

manganese « 208 

mercury 210 

silver 210 

Zinkenite,  analysis  of 260 

Zircon  "  .  253 


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Wellington's  Location  of  Railways. 8vo,  5  00 

8 


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Wolff's  Windmill  as  a  Prime  Mover 8vo,  3  00 

HYDRAULICS. 
"WATER-WHEELS — WINDMILLS — SERVICE  PIPE — DRAINAGE,  ETC. 

(See  also  ENGINEERING,  p.  6.) 
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(Trautwine) 8vo,  2  00 

Bovey  's  Treatise  on  Hydraulics 8vo,  4  00 

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Fuerte's  Water  and  Public  Health 12mo,  1  50 

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Wilson's  Irrigation  Engineering 8vo,  4  00 

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Wood's  Theory  of  Turbines Svo,  2  50 

MANUFACTURES. 

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WOOLLENS,  ETC. 

Allen's  Tables  for  Iron  Analysis Svo,  3  00 

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Bollaud's  Encyclopaedia  of  Founding  Terms 12mo,  3  00 

The  Iron  Founder 12mo,  250 

"          "       "          "        Supplement 12mo,  250 

Booth's  Clock  and  Watch  Maker's  Manual 12mo,  2  00 

Bouvier's  Handbook  on  Oil  Painting 12mo,  2  00 

Eissler's  Explosives,  Nitroglycerine  and  Dynamite 8vo,  4  00 

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9 


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"        Handbook      for      Chemists      of      Beet       Houses. 

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The  Lathe  and  Its  Uses 8vo,  6  00 

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Walke's  Lectures  on  Explosives 8vo,  4  00 

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MATERIALS  OF  ENGINEERING. 

STRENGTH — ELASTICITY — RESISTANCE,  ETC. 
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Baker's  Masonry  Construction 8vo,  5  00 

Beardslee  and  Kent's  Strength  of  Wrought  Iron 8vo,  1  50 

Bovey's  Strength  of  Materials 8vo,  7  50 

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Byrne's  Highway  Construction 8 vo,  5  00 

Carpenter's  Testing  Machines  and  Methods  of  Testing  Materials. 

Church's  Mechanics  of  Engineering — Solids  and  Fluids 8vo,  6  00 

Du  Bois's  Stresses  in  Framed  Structures 4to,  10  00 

Hatfleld's  Transverse  Strains 8vo,  5  00 

Johnson's  Materials  of  Construction 8vo,  6  00 

Lanza's  Applied  Mechanics 8vo,  7  50 

"       Strength  of  Wooden  Columns 8vo,  paper,  50 

Merrill's  Stones  for  Building  and  Decoration 8vo,  5  00 

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Pattbu's  Treatise  on  Foundations 8vo,  5  00 

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Thurston's  Materials  of  Construction 8vo,  5  00 

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Tlmi'ston's  Materials  of  Engineering 3  vols.,  8vo,  $8  00 

Vol.  I.,  Xou-metallic  8vo,  200 

Vol.  II.,  Iron  and  Steel 8vo,  3  50 

Vol.  III.,  Alloys,  Brasses,  and  Bronzes 8vo,  2  50 

Weyrauch's  Strength  of  Iron  and  Steel.    (Du  Bois.) 8vo,  1  50 

Wood's  Resistance  of  Materials 8vo,  2  00 

MATHEMATICS. 

CALCULUS— GEOMETRY — TRIGONOMETRY,  ETC. 

Baker's  Elliptic  Functions 8vo,  1  50 

Ballard's  Pyramid  Problem 8vo,  1  50 

Barnard's  Pyramid  Problem 8vo,  1  50 

Bass's  Differential  Calculus 12mo,  4  00 

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Chapman's  Theory  of  Equations 12mo,  1  50 

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Comptou's  Logarithmic  Computations 12mo,  1  50 

Craig's  Linear  Differential  Equations 8vo,  5  00 

Davis's  Introduction  to  the  Logic  of  Algebra 8vo,  1  50 

Halsted's  Elements  of  Geometry t..8vo,  175 

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Johnson's  Curve  Tracing 12mo,  1  00 

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"  Unabridged. 

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Mahan's  Descriptive  Geometry  (Stone  Cutting). 8vo,  1  50 

Merrimau  and  Woodward's  Higher  Mathematics 8vo,  5  00 

Merriman's  Method  of  Least  Squares 8vo,  2  00 

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"        Linear  Perspective 12mo,  1  00 

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11 


Wan-en's  Plane  Problems  12mo,  $1  25 

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"        Problems  and  Theorems 8vo,  2  50 

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Wood's  Co-ordinate  Geometry 8vo,  2  00 

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TEXT-BOOKS  AND  PRACTICAL  WORKS. 
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Baldwin's  Steam  Heating  for  Buildings 12mo,  2  50 

Benjamin's  Wrinkles  and  Recipes 12mo,  2  00 

Carpenter's  Testing  Machines  and  Methods  of  Testing 

Materials 8vo. 

Chordal's  Letters  to  Mechanics 12mo,  2  00 

Church's  Mechanics  of  Engineering 8vo,  6  00 

"        Notes  and  Examples  in  Mechanics 8vo,  2  00 

Crehore's  Mechanics  of  the  Girder 8vo,  5  00 

Cromwell's  Belts  and  Pulleys 12mo,  1  50 

Toothed  Gearing 12mo,  150 

Compton's  First  Lessons  in  Metal  Working 12mo,  1  50 

Dana's  Elementary  Mechanics 12mo,  1  50 

Dingey's  Machinery  Pattern  Making 12nio,  2  00 

Dredge's     Trans.     Exhibits     Building,      World     Exposition, 

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Du  Bois's  Mechanics.     Vol.  I.,  Kinematics 8vo,  3  50 

Vol.  II.,  Statics 8vo,  400 

Vol.  III.,  Kinetics 8vo,  350 

Fitzgerald's  Boston  Machinist 18mo,  1  00 

Flather's  Dynamometers ." 12mo,  2  00 

Rope  Driving 12mo,  200 

Hall's  Car  Lubrication 12mo,  1  00 

Holly's  Saw  Filing 18mo,  75 

Johnson's  Theoretical  Mechanics.      An  Elementary  Treatise. 
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Jones  Machine  Design.     Part  I.,  Kinematics 8vo,  1  50 

Part  II.,  Strength  and  Proportion  of 
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Lanza's  Applied  Mechanics 8vo,  7  50 

MacCord's  Kinematics 8vo,  500 

Merriman's  Mechanics  of  Materials 8vo,  4  00 

Metcalfe's  Cost  of  Manufactures 8vo,  5  00 

Michie's  Analytical  Mechanics 8vo,  4  00 

12 


Mosely's  Mechanical  Engineering.     (Maban.) 8vo.  $5  00 

Ricbards's  Compressed  Air 12mo,  1  50 

Robinson's  Principles  of  Median  ism 8vo,  3  00 

Smith's  Press-working  of  Metals 8vo,  5i  00 

Tbe  Latbe  and  Its  Uses 8vo,  6  00 

Thurstou's  Friction  and  Lost  Work 8vo,  3  00 

Tbe  Animal  as  a  Macbine ,  12mo,  1  00 

Warren's  Macbine  Construction . .  .2  vols.,  8vo,  7  50 

Weisbach's  Hydraulics  and  Hydraulic  Motors.    (Du  Bois.)..8vo,  5  00 
Mechanics    of    Engineering.      Vol.    III.,    Part  I., 

Sec.  I.     (Klein.).... 8vo,  500 

Weisbacb's  Mechanics    of  Engineering.     Vol.   III.,    Part  I., 

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Weisbacb's  Steam  Engines.     (Du  Bois.) , 8vo,  500 

Wood's  Analytical  Mechanics 8vo,  3  00 

"      Elementary  Mechanics 12mo,  125 

Supplement  and  Key 1  25 

METALLURGY. 

IRON— GOLD— SILVER — ALLOYS,  ETC. 

Allen's  Tables  for  Iron  Analysis 8vo,  3  00 

Egleston's  Gold  and  Mercury 8vo,  7  50 

Metallurgy  of  Silver 8vo,  7  50 

*  Kerl's  Metallurgy — Copper  and  Iron 8vo,  15  00 

Steel,  Fuel,  etc 8vo,  15  00 

Kunbardt's  Ore  Dressing  in  Europe 8vo,  1  50 

Metcalf  Steel— A  Manual  for  Steel  Users 12mo,  2  00 

O'Driscoll's  Treatment  of  Gold  Ores 8vo,  2  00 

Thurstou's  Iron  and  Steel 8vo,  3  50 

Alloys 8vo,  250 

Wilson's  Cyanide  Processes 12mo,  1  50 

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Barringer's  Minerals  of  Commercial  Value oblong  morocco,  2  50 

Beard's  Ventilation  of  Mines 12mo,  2  50 

Boyd's  Resources  of  South  Westep  Virginia 8vo,  3  00 

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Brush  and  Peufield's  Determinative  Mineralogy .Svo,  3  50 

Chester's  Catalogue  of  Minerals 8vo,  1  25 

paper,  50 

"       Dictionary  of  the  Names  of  Minerals 8vo,  3  00 

Dana's  American  Localities  of  Minerals Svo,  1  00 

13 


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"      Mineralogy  and  Petrography     (J.D.) 12uio,  2  00 

"      Minerals  and  How  to  Study  Them.     (E.  S.) 12mo,  1  50 

"      Text-book  of  Mineralogy.     (E.  S.) Svo,  350 

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Egleston's  Catalogue  of  Minerals  and  Synonyms Svo,  2  50 

Eissler's  Explosives — Nitroglycerine  and  Dynamite Svo,  4  00 

Goodyear's  Coal  Mines  of  the  Western  Coast 12mo,  2  50 

Hussak's  Rock- forming  Minerals.     (Smith.) Svo,  2  00 

Ihlseng's  Manual  of  Mining Svo,  400 

Kuuhardt's  Ore  Dressing  in  Europe , Svo,  1  50 

O'Driscoll's  Treatment  of  Gold  Ores Svo,  2  00 

Rosenbusch's    Microscopical    Physiography   of    Minerals    and 

Rocks.     (Iddiugs.) Svo,  5  00 

Sawyer's  Accidents  in  Mines Svo,  7  00 

Stockbridge's  Rocks  and  Soils Svo,  2  50 

Walke's  Lectures  on  Explosives Svo,  4  00 

Williams's  Lithology Svo,  3  00 

Wilson's  Mine  Ventilation 16ino,  125 

"        Placer  Mining 12mo. 

STEAM  AND  ELECTRICAL  ENGINES,  BOILERS,  Etc. 

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Clerk's  Gas  Engine t 12mo,  400 

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MacCord's  Slide  Yalve Svo,  2  00 

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Meyer's  Modern  Locomotive  Construction 4to,  10  00 

Peabody  and  Miller's  Steam  Boilers Svo,  4  00 

Peabody's  Tables  of  Saturated  Steam Svo,  1  00 

Thermodynamics  of  the  Steam  Engine Svo,  5  00 

Valve  Gears  for  the  SteannEngine Svo,  2  50 

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Thurston's  Boiler  Explosion 12mo,  1  50 

14 


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and  Theory 8vo,  7  50 

4t  Manual  of  the   Steam  Engine.      Part  II.,    Design, 

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2  parts,  12  00 

"           Philosophy  of  the  Steam  Engine 12mo,  75 

"          Reflection  on  the  Motive  Power  of  Heat.    (Caruot.) 

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Fisher's  Table  of  Cubic  Yards Cardboard,  25 

Hudson's  Excavation  Tables.     Vol.  II 8vo,  1  00 

Johnson's  Stadia  and  Earthwork  Tables 8vo,  1  25 

Ludlow's  Logarithmic  and  Other  Tables.     (Bass.) 12mo,  2  00 

Thurston's  Conversion  Tables 8vo,  1  00 

Totten's  Metrology 8vo,  2  50 

VENTILATION. 

SIEAM  HEATING — HOUSE  INSPECTION — MINE  VENTILATION. 

Baldwin's  Steam  Heating 12rno,  2  50 

Beard's  Ventilation  of  Mines 12mo,  2  50 

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Gerhard's  Sanitary  House  Inspection Square  16mo,  1  00 

Mott's  The  Air  We  Breathe,  and  Ventilation 16mo,  1  00 

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15 


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Barnard's  The  Metrological  System  of  the  Great  Pyramid.  .8vo,  1  50 

Davis's  Elements  of  Law . 8vo,  2  00 

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*  Wiley's  Yosemite,  Alaska,  and  Yellowstone 4to,  3  00 

HEBREW  AND  CHALDEE  TEXT=BOOKS. 

FOR  SCHOOLS  AND  THEOLOGICAL  SEMINARIES. 

Gesenius's  Hebrew  and   Chaldee  Lexicon  to  Old   Testament. 

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Ruddiman's  Incompatibilities  in  Prescriptions .8vo,  2  00 

.  Steel's  Treatise  on  the  Diseases  of  the  Ox 8vo,  6  00 

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